Wizard cons can be very profitable for vendors; and I think your product would do very, very well there. They attract large crowds for the most part*, so I think you'd definitely be able to make up the cost of the booth rental.
However, that being said, since they approached you, I would try and negotiate with them as far as booth cost. The worst they will say is "no can't do", but you do have the upper hand in the situation since they sought you out to ask you to be a vendor. *Plus the Raleigh one(which i'm asssuming by taking a quick look at your shop is the one they asked you to be a vendor at) is new, and I would definitely add in that a $600 booth is sort of pricey when you don't know if the convention is going to have much success, as they haven't really tested the waters there, yet.