Leveling can be hard when it comes to making money. Let me rephrase that, Leveling can be hard when it comes to making money if you do not make the best of what you get while leveling.
You will be killing animals left and right. Some animals drop meat while other seems to be meatless animals. The ones with the meat will help you level your cooking but once you passed the need for that meat you will usually do one of two things with it. Cook it anyway because you use that food or vendor it.
Cook it if you need it, it is serving a purpose for you. If you do not need it do not vendor it. Save it up and sell it. Meat, even more so lower level meats, sell for quite a bit on the Auction House. Maybe not a lot for a level 80 but while leveling selling meat for 50 or 60 silver each can be a small fortune and surely more then vendoring it for a few copper.
Nearly all meats sell for a nice amount and some even go as high as 2-4 gold each. The best time to list your uncooked food for sale is early Saturday morning or even late Friday night. The reason for that is that many people like to level alts on the weekend and people with money that level alts never take the time to worry about getting the materials normally, they power level everything. If you have giant eggs left over that you do not need it is not unheard of for someone to buy a stack of them for 100 gold on a Saturday because they needed it for power leveling their cooking on an alt.
Some meats are even worth farming if you are in an area anyway. If the kill is quick for you and it is close to you and you are still getting good experience from it, go for it and kill it. It is win/win. You are there anyway, it was a quick kill that granted decent experience and it could very well drop something you can sell for a pretty penny.
A list of some of the meats I have noticed sell for those most on the AH: (may differ on different servers)
Jaggal Clam Meat
Zesty Clam Meat
Tender Clam Meat
Those are some of the best while leveling that I have seen. However anything that can be sold is good to make an effort to sell. White items that are food are always going to be needed by someone that is trying to level cooking and do not think because the meat is not on any power leveling guide that it is not worth selling. Power leveling guides bring people out to farm those things exactly to sell for a high price. If I power level something I always go against the guide.
There are always secondary things you can do to level and you will get the meats cheaper. Lets say something requires bear meat and all the guides suggest that. Bear meat will be 1g each on the market but you could also get red wolf meat to level the same skill set but the guides do not mention that. Sometimes you will see the red wolf meat on the market for less then the bear beat and by that instead. So if you see bear meat for 1g and wolf for 90s you will buy the cheaper ones to power level. So that means even if red wolf meat is no on any leveling guides does not mean you can not make a nice amount selling it. Everything sells.
Once in Northrend nearly all meat is worth more then vendoring as many of the lower level meats you will come across are still used for end game like Rhino Meat, Chunk of Mammoth and other goodies that you first get the moment you step foot into Northrend. Never ever throw away any food anywhere. Sell it if you do not need it for yourself.
Just recently while leveling I decided to sell off some meat I had saved up on one of my characters that I was using to level cooking with. After I was done I sold all the extras, was not much, less then one 20 slot bag worth in my bank, and I ended up with 320 gold.
320 gold just from left over meat, at level 55 now, so when I get to outlands I got my mount, and some, paid for already from saving meat and not throwing it away. If you are making your first character this can be make or break. If you already have 5 80s and tons of gold, this is still worth doing. It takes no time what so ever to list something instead of throwing it away. Even if you are already at the gold cap would you just throw 320 gold away? I guess not. Making money by saving food should be done by everyone.