It all depends on your server is the answer.
But in general, if you really want to make money, you have to track how much your missions are, the prices of items you can make on the gym, and the prices of the raw materials on the gtn. If you don't, you may find yourself spending more to sell for less, which is a money LOSING proposition. On my server, I've seen the prices of raw materials fluctuate so much that sometimes it is more profitable to only make things for myself and sell the raw materials. Other times, the raw materials are so over-priced no one buys them and you're better off crafting things to sell. You also have to keep track of the cost or discovery missions. Too many people buy these not realizing they are taking a loss on the value of the materials they have gotten.
If you're just starting off, you don't have to get every recipe from the trainer. It costs a lot when you get from 300-400. If you want to get to biochem 400 cheaply, find the cheapest thing you can make with the fewest materials and only craft oranges (give you more biochem points to craft). Just make that one thing til you can get the next level of cheap recipe and only get that one. You will get to 400 with few recipes but once there, you can make more money selling the best cost to price item on the gtn or selling the most profitable raw materials. It also helps a lot for you to utilize the strengths of your companions. I only have Doc craft my biochem because he has a biochem crit bonus. Nadia has a diplomacy crit so she does diplomacy missions.
Also, keep in mind, that some raw materials can be bought cheaper then you sending companions out to get them on missions.
Find your niche. Own your niche. In general, I only sell a few items. I know the different people who make those items and how much they try to sell them for. I know the market for the materials that make those items too. Do what's profitable. Always be in the look out for new opportunities. While leveling another toon, I noticed there were a of items for that class/level that no one makes. I'll be experimenting to see if that is an oversight or because no one buys them. Lastly, keep in mind the law of supply and demand. Don't flood your market with a million items or you will see the prices drop.
Good luck.
Dirr/Colja on Harbinger