We build our walls out of a wooden frame (2 x 8; 3 x 8) and streach sheets over them staple then paint. after two or three coats of paint the sheet gets hard almost like wall board but much lighter. We have almost 200 now. We build a few new ones every year. Sheets and paint can be gotten for free. Call the paint stores for "oops paint" and call motels for their old sheets. I use 2 X 1 X 8' furring strips that cost about $2. each to make the frames. They are light, easy to move, can be painted different for different years. when we put the pannels up we leave gaps that are just hung with a sheet. That allows the actors to move back and forth between scares.
You paint old sheets black and hang on walls and ceilings to cover. Paint the sheet before you hang it or it will bleed through to the wall. We have a couple of hundred painted sheets and add some every year.

We have not had to pay for any sheets. Most of the paint has been donated. One paint store was willing to put the tint for black into their oops paint for me.

A bonus is that latex paint won't burn. although the wood frame and the material under the paint will. I buy New York Invisishield and spray everything down every year. 5 gallons is around $250. When we are haunting with only about 3,000 sq. ft., the 5 gallons lasted for two years. Last year with 10,000 sq. ft we used the whole 5 gal.

Keep a staple gun and some screws handy as someone always gets scared and runs through a wall pannel. repairs are quick and easy however.