I have some expierence with these shows and maybe can give some good advice.
I have had booths with local jobbers before at all kinds of these car show and basically have found from a manufacturers point of view, zero bang for the buck.
Watching the jobber, not so sure he is getting any bang for his dollar either. I can recall one new customer for one jobber with all the shows over the years I have done.
The problem is, 99% of the people at these car shows don't paint and just want to ask a 1000 questions.
For a body shop! Some of these shows if local or something like the GTO nationals can be a gold mine. I have had a booth and did classes at two GTO nationals and really other then enjoying the cars not worth the time BUT both of those times there was a restoration shop with an inside booth and he cleaned up both times with potential customers and this last show I would guess he got at least 7-8 and know this only, because I was close to his booth.
Even for the body shop, I think the "type of show" would be critical, the local drive in of 300-350 car show we have the 3rd week of month, would be a waste of time but good-guys and major chevy or gto type shows could be a money maker for a body shop, from what i see.