The reason why haunts are open on the weekends is because it is more efficient for customers as well as actors. A lot of people work during the week and if a haunt is open until midnight, most actors would have to leave before then depending on work/school and most customers would stop showing up after 10pm. Another reason why many haunts up their price is because of the demand, depending on how popular they are, the cost of electricity it takes to run their location, payment for the actors if they do pay, and then the rest goes for new props/wood/etc for the next season. Every dime they make goes to the attraction.
This is just my perspective being that I work at a haunted house as both an actor and a decorator.