Tarantulas can range in price from a few dollars to a few hundred dollars. I would suggest doing a bit of online research on the species that you are interested in and what they are currently being sold for. This can give you some idea of what you can expect to pay at a show. A good place to start might be here in the "For Sale" section.
As for the size usually seen for sale, this can vary between 1/4" or smaller to upwards of 9" plus for some of the larger species.
The trend I've seen is that most of your reptile dealers will have a few of the hobby standards that are large WC specimens at a good price. Your invert dealers will have a few of the larger specimens, but will mostly have slings available for sale. I'm sure it varies from dealer to delear, but that is what I've seen and how I do it. Your best bet would be to find out what dealer is going to be at a particular show and ask him/her what they are bringing.
A few months back, I purchased 2 A. seemani's for 9.99 a piece. After I got them home, and fattened the skinny little things up, I now have 2 females..
It was yet another "pity" buy for me. They were in deli cups far too small, and were so tiny, skinny I almost threw a fit. I wanted to sex them, but the dealer had a small cow at the thought of me taking them out.
I may be wrong, but I think they were so skinny so as to keep them calm looking. When I set them up in their enclosures, they both drank for quite awhile. After about 4 days, their typical A. seemani temperament came out..
Peace, light & eternal love...