I think everything in the above posts is accurate but there are some other factors that you should also think about. First, there does not seem to be (as far as I am aware) an industry standard for specifying the C-rating of lipo packs. So one manufacturer's 30C 2200mAh 3cell may not be as good as another's. Second, you generally get what you pay for, not always, but generally. So look to buy half decent or well reviewed brands. Lastly, get a wattmeter or at least, a decent battery tester. You will waste your money buying a decent brand, high C battery if all you do when you get it is overdraw current or overdischarge it.
Best of luck.