As with anything else, the market will even things out.
Haunts should charge what the market will bear, and the market differs from location to location.
I don't run a professional haunt, yet, but I've seen the prices associated with doing so. It's not cheap.
If your haunt is worth $20 or $30 to the patrons that go through it, then that's what you should charge.
There's a sweet spot that each haunt needs to find: a balance between what people are willing to pay and what will allow you to afford to continue running and improving the haunt.