I am an herbalist/alchemist, and I make a reasonable amount of money on it. I'm currently level 30, though, so I'm not sure how it will go later on in the game. That said, I'm pretty confident that I will be able to pay for my mount at level 40. I made 15g just in one level (29-30). The trick is to learn exactly which potions sell and which don't, as well as how much they sell for: doesn't make sense to lose money because you can only sell it for less than you spent on it (if you spend any money). Also, as others have said, watch the auction house, and don't flood the market with 20 stacks of 5 Swiftness Potion for the lowest possible price. Only sell one stack at a time, and keep the rest in the bank, and only sell it when there aren't already a lot of that item. This takes patience and bank space.
Then, only make the potions you need (I'm a priest and therefore, make lots of mana potions for myself) and the ones that sell well (Swiftness Potion does well, as does Shadow Oil, at least on my server, and I've also had success with some others that I can't remember offhand). If there are any herbs that aren't used in those potions, sell those herbs in stacks of 20. I sell peacebloom, silverleaf, mageroyal, and bruiseweed. While I could use the bruiseweed in the Healing Potion (which does sell), I don't because it requires briarthorn, and briarthorn is better off in the Swiftness Potion (which sells better than the Healing Potion). As others have said, you'll have to check yourself to see what is actually selling well, and it might take a couple of false starts.
Also, as others have said, take up fishing:
1. Oily blackmouth, firefin snapper, and stonescale eels are all needed for certain alchemy recipes, and they might do well. If they don't, you can sell the fish to other alchemists. From what I've heard, they do pretty well.
2. Anytime you are fishing in a pool/school, you have a chance of getting stranglekelp or chests with green items, potions, cloth (usually bolts), or leather. I've made a lot of money from items I've fished out of chests. Stranglekelp is also used in a number of potions, such as the Mana potion, the water breathing potion, and the swim speed potion. It's a little difficult to get, since it only grows underwater. Having a chance to fish it up makes it a little easier.
Also take up cooking. Cooking and fishing go together, and you can get a decent amount of money for Smoked Sagefish. The recipe is bought from most cooking supplies vendors, but since it has a really good mana regeneration effect, it does pretty well. I don't sell anything else I cook, but I've done well with the sagefish. Plus, I've found it easier to find sagefish schools than blackmouth or snapper schools, since the latter are only found in salt water, while sagefish is in fresh water. The only exception I know of is the oil spills at the Venture Company. Then, later, you can fish greater sagefish and make sagefish delight. I haven't sold any of that yet, because I'm hoping to use it myself, but I'm certain that it would do at least as well as the smoked sagefish, and probably better. Again, all schools give a chance for a chest or stranglekelp.
Cloth:
I gave up on first aid, aside from the anti-venom (which I find useful), because I can make a lot of money on cloth, especially wool cloth. On my server, a stack of 20 wool cloth goes easily for 2g, and I'm probably underpricing it a bit for my server. I went up to 2g20, and it sells just as fast. So, I recommend saving any cloth you get and selling it.
Those are the tips I have. I don't really feel like I have a good financial sense, I've just read a lot of similar advice, and customized it for alchemy. I hope this was helpful