It all depends on where these conventions are held. Hotel ballroom or conversion center, the price for a space for a booth can get very expensive. Because you may profit from that space they want their share for providing it, space is finite.
A normal size booth space on McCormick Place in Chicago for some events is $25k. I'm sure there are MUCH cheaper places as McPlace is an extreme money pit. If you're not already established with money, good luck on that gamble to get enough biz to pay for it.