I suspect we're not all talking about the same things. The exhibitions I've typically been entering are put on by art clubs or non-profit associations, run mostly by volunteers, not commercial galleries. The entry fees typically run 10 to 40 dollars, and there are prizes in the $25 to $500 ballpark. At least one is up for a month in a relatively upscale area and does sell some work. I look at these shows as a lottery -- but you have to bring a picture or two.
Charging admisson might work in the case of a long running, well established show, but I think for less known shows it would guarantee they would stay less known or dry up altogether.
In any event, each of us has to weigh the cost, the exposure, the sales potential, the opportunity to see our work next to that of others, etc. and how it fits our goals. If you like it do it, if not, don't. I know all too well how much work and money goes into mounting a relatively trivial show, in fact, I'm presently in the middle of the prep for one.