Wondering how to make some big bucks on transmuting here is a little guide have fun with it.
Transmuting Mithril To Truesilver
Let's say you've just recently put together your Alchemist's Stone and got to Artisan level. After heading to Gadgetzan, the first transmute you'll gain access to is the Mithril->Truesilver one. (There's also the Iron->Gold one, that's really good for nothing so you might as well skip buying it). The only reason to use this one is for cheap leveling of your alchemy skill, to get to 275 skill. You will make a profit with it too, but it won't be very large.
Consider investing in a stack of Mithril bars. Make sure you haggle for the price, get it as cheap as you can. When you start burning through them, sell the resulting bars one-by-one at the AH. Unless you're selling on a bad day or something, you will more than double your per-bar investment in mithril. Say you bought your stack for 10-15 silver per bar - well, you'll be selling your truesilver for 40-60 silver a piece. At that rate it takes about one week to pay off the initial investment, the rest is money for nothing, in your pockets. Won't amount to a whole lot, but in your early levels you need all the help you can get - and besides, you will get the occasional skill-up while at it, too.
Once you reach level 275, you can learn the Arcanite transmutation. This recipe will last you a good long while - in fact, unless you luck out and find the Undeath->Water essence transmute somewhere, you'll be using it right up to level 60 when you have completed the Scholomance ghost quest chain.
The price of Arcanite as well as arcanite transmutes has been heading downhill for a while, as is the case with all things in WoW. Like most people, I started out by offering transmutation services for a fee of 5 gold, but soon that price started lowering as more alchemists entered the market, and on my server it's now settled in at 3 gold. But I got to thinking... I'm wasting a lot of time advertising my services in Ironforge and putting up with annoying customers - for what? A few gold every few days? Considering my time and effort spent advertising and bargaining, I knew I had to find an easier way out.
By this time I had a few gold in my pocket already, and even though selling my own bars meant tying up almost 30 gold of my cash, risking unsuccessful sales in the AH and paying 5% for even the successful ones, none of these things were a real obstacle to me. I bought a stack of Thorium Bars for a few gold, and started browsing for arcane crystal prices. I quickly learned that there was quite a gap between the price of a crystal and the price of the finished Arcanite bar: A crystal would go for 19-22 gold, whereas the bar prices ranged from 25 to almost 30 gold.
By entering the bar sales business, you'll bring in an average of at least 6 gold profit per sale - which should be twice the amount you'd get from selling the service alone, and without the hassle of haggling and advertising.
Undeath to Water
I got lucky and found the Undeath->water transmute recipe in Un'Goro, so I don't know what this is worth in the AH. Probably a lot, since it's a world drop recipe. If you find it, it's a gold mine. If you see it in the AH, you'll have to weigh the cost/future profit carefully, because you'll probably switch to doing a different transmute later. It won't be wise to spend obscene amounts on this recipe.
Assuming you have it one way or another, here's the deal. Essence of undeath is a somewhat useless and one of the more common essences, found often in Stratholme and Scholomance as well as other mobs. I could buy these essences for anything from as low as 50 silver to 1,5G from the AH. To my delight, Essences of Water are far more difficult to attain and are used in a number of high level crafted items. Their AH price fluctuates as farmers have better and worse terms, but it has stayed in the 7-10G region for a couple of months now on my server.
So, from this transmute you'll earn 6-8 gold or more per transmute - the same amount you got for arcanite - with one major exception: The cool down is only 1 day. Congratulations, you are now making 250 gold per month doing absolutely nothing. But if you want to double that for just 2 hours per week, read on.
Water to Air
Complete the quest chain in Scholomance given to you by Eva Sarkhoff, the ghost outside the entrance, you gain an item that allows you to interact with the other ghosts on the island. Among them you will find a vendor that will sell you the Water->Air transmute recipe (binds when picked up).
Essences of air are found in Silithus, but their drop rate is highly irregular. They are also used in various rare and epic crafted items. Thus they have earned their AH price of approximately 15-20 gold on my server. By buying Water essences and selling Air essences, you will make 7-8 gold or more per day - only slightly more than by doing Undeath->Water. Prices may vary on your server, of course. This is the easy way of doing things.
If you are of the grinding mindset, you can of course grind your own essences to double your profit. This is not difficult, if a bit tedious.
You can find Essences of Water from high-level water elemental mobs. The two convenient places I frequent are: Felwood, or Eastern Plaguelands. Both locations are heavily farmed, and with the advent of the new warlock quests in Felwood that happen to be very close to the elemental area, the small area is often packed with 3 or more players competing for the spawns in the relatively small area. The Plaguelands areas are also farmed, but instead of one there are two, and the other one is significantly large which allows for many players to grind there without stepping on each other's toes too much. It's of course more fun to do it when you're alone at it, so you don't have to wait around and compete for the spawns.
The drop rate of these essences is not too low, but low enough to make you really suffer on an unlucky day. Sometimes you can go for an hour without seeing the first sign of any essences, but other times you get two on consecutive kills. Farming random drops is like this, and it's often boring, and the elementals don't really drop anything else of value. Almost no greens, just enchanted water and the occasional elemental water or globe of water. I guess this is why the price of the essences remains fairly high.
But if you're persistent, and have the tenacity to grind for 2 hours straight, you will probably be rewarded with at least 4-6 essences, if not more. For about 2 hours worth of grinding, you're set for a week of transmuting at 15-20G a sale.
There's your 500G a month, 2 hours per week. :)
As with everything, expect these prices to lower over time. I've been doing this for about a month now, with no significant drop in profits (Air essence prices have gone down, but Water essence prices have gone up). By the end of the year I don't think it'll be as profitable, but considering the time spent it should still be well worth it.
The transmutes I outlined above are arguably your best path to 300 skill, and your ultimate moneymakers as far as transmutes go. There are other transmutes available, but they really serve little purpose from a moneymaking perspective at least until Blizzard adds new crafted items or quests that shift the value of various essences and minerals.
In particular I recall once reading a funny comment about the Air->Fire transmute: Someone compared its usefulness to that of an 'Arcanite->Copper' transmute... And really, that's what it sounds like to me as well ;).
However, there is one alternative to consider, and you can do your own math on your server to see if it's worth doing. Thank you to Squigipapa on the forums for bringing up this idea.
Earth to Water (to Air)
If Water essences are too heavily farmed on your server for you to step in and get your own in a reasonable amount of time, you might consider farming Essences of Earth. First of all you need to be friendly with the Timbermaw Furbolgs to purchase the Earth->Water transmute recipe from then.
You can then grind Essences of Earth in Silithus, where rock elementals are plentiful and not quite as farmed as Water Elementals. By transmuting them to Water, and then to Air the next day, you may be able to turn up a nice profit, although the time involved in the process does leave you with only half the gold you could make from directly grinding Essences of Water. Alternatively you could also sell the Water Essences depending on how the essence market is working out on your server. Just some food for thought.