Be a miser.
I rarely buy anything, and my mother gives me money for food so I can save my personal money. I can't remember how I accumulated it over the years, but it's there because I think before I buy. Is it something I really need, or can I do without it? A lot of times you can talk yourself out of buying things you don't need by taking a moment to really think about it.
Specifically relating to costuming, save scraps from previous projects. You never know when they will be useful.
I sold artwork at the con, which covered the expenses of going and it was probably enough to cover the cost of my costume as well, since I ended up only making one.
Basically, find something you're good at that people are willing to pay for. Everyone has some talents and skills they can use. If you're good at playing an instrument, stand on a street corner in a shopping district and play. Offer to do odd jobs for your neighbours. Be sure to be realistic about your abilities, though.