It really depends on the artist and the work you are having done.
If they are a really popular artist with a lot of bookings they are unlikely to drop their prices.
That said if the artist is having a slow day and you want a fast piece you may get a deal. Because ppl are more likely to get work done from a busy artist than not.
You can always ring the artist and ask for a quote and book in a slot at the convention.
Take Care
Matthew