At anime conventions is stuff really over priced?

No, it's not. It's like any science fiction convention - you'll pay retail for it unless you're looking at rare collectibles. Save up whatever you like, make sure you have enough for the hotel (if you're staying overnight) and the admission fee. The dealers room is a lot of fun. They do take checks and usually credit cards. Unless you're looking for home made items. The people who make those will charge what they need to to cover their costs and make a profit.

Do your research now online to find out what average prices are for the items you want so you will know what's a deal and what's overpriced.

If its anything like the renaissance fair or other events then yes, everything will be stupidly overpriced.