From the player that pretty much has the market on 1-300 or 300-375 profession leveling guides, come an interesting analysis of the cost of leveling the professions. Interesting to note that enchanting is the most expensive profession based on buying mats from the AH. Granted, I know all you enchanters get most of your mats from disenchanting items, but it still bring up an interesting point.
I still don't have my flying epic mount. I've practically given up on farming for gold because I've found it mostly un-fun. I'll use my enchanting to get some gold here or there. I run enough instances that I break even at worst after repairs. I had always wanted to get the flying epic mount, and still do, but it's become less of a priority at the moment. From the looks of that analysis, I probably spent enough during the leveling process of tailoring and enchanting that I could have had a flying epic mount. But, as a guildie said, you can't take an epic mount into a dungeon.
So, what does that mean? For a mage, having tailoring and enchanting as your two professions will almost always give you a leg-up. At 375, you can craft all of the Spellfire set including the [Spellfire Robe]. Each piece by itself is amazing considering you don't have to set food in any dungeon. But then you look at the 3-piece set bonus. Plus spell damage up to 7% of your Intellect. Being that Intellect is one of our major stats, it's very nice to have an increased reason to have that stat. And, in case you didn't notice, all of those items are BoP. So, while it may cost you 1,381g to get tailoring up to 375 (not including the gold or mats to make the Spellfire set), you can bring that into a dungeon. So, tailoring for a mage, while expensive, is a really good choice.
What about enchanting? When I first started leveling enchanting, I did it because I wanted to make myself a wand. Little did I know that the enchanting wands didn't go very far, and I'd usually replace them with dungeon drops moments after I was able to actually craft them. It's been expensive, and a small part of me misses that money. But overall, I believe I've spent enough time advertising in the trade channel to at least make the hurt not so bad. So now that I'm 375, what now? Well, with the release of the Burning Crusade, the only enchant that non-enchanters can't get is enchant to rings. So that means that because I'm an enchanter, I can have an extra 24 spell damage (12 per ring) more than other non-enchanters can have. Would I suggest dropping 4,842g so you can get 24 more spell damage? Probably not. But at least it's a semi-sane justification for all the money I've put in. Don't tell me otherwise... I'm contend in my own little coma.