In addition to money, make sure you have enough time for the costume you pick, and don't put everything off until the last minute. It's cheaper to make your own costume than to commission it or buy a premade one, but if you don't have the time it won't turn out well. It's better to have one really great costume than a bunch of half-assed ones. It's also a good idea to start out simple, to give yourself practise and so that if you mess up you haven't lost as much money.
And if you aren't sure about something, like a certain fabric, ask someone about it, because you don't want to buy something and find out it doesn't work. Making your patterns before you buy the fabric is also a good idea. Buying too much will cost you, but you don't want to buy too little. For cosplay, you probably don't need to buy a really expensive fancy fabric. Normal fabrics are usually cheaper and easier to work with, and they often photograph better.