It depends on the convention.
For example, I recently went to ANEXT, which is a medium sized con. I only spent about $160 between gas money, ticket, food, etc. I also didn't have to pay for a hotel, so that helped.
However, for Otakon coming up, I need to pay $150 for the room and $70 for the ticket itself. Then I need to buy and put together all my cosplay stuff (I've spent $150 on wigs and make up supplies so far). And then I need at least $100 for food and merchandise.
If you cosplay, expect to be throwing away a lot more of your money. It's not a cheap hobby.
But if you're just attending, a big convention like that will cost you $300-$400 easily.