I will be making walls out cardboard from refridgerator boxes, and a little paper mache for an inside haunt this year. I plan on using carpet tubes for framing. Very cheap. http://www.johnnyspage.com/walls.htm
Cheap for me is the cardboard (I get refridgerator cardboard boxes from a friend in the appliance industry) and carpet tubes for the framing (I have been slowly stocking up on these as I drive by a carpet store on the way home from work). I am not absolutely sure it will work yet, but since the walls will be inside, and I will be able to support the tubes at the top against the ceiling, I think it will. I will certainly document the results to pass on.
You are right madmax, the framing is tricky part. It took a while before I had the idea to try the carpet tubes, I am sure there are other alternative out there, just need to keep your eyes open as you already do so well.
I made "some" of my wall sections for my "inside" haunt from cardboard and I bought 2x4's and cut them in half with a table saw to make 2x2's but not exactly but 1 2x4 is less than 2 2x2's so it saved $$$. I put one brace in the middle across the frame. They held up very well and were light weight compared to the other all wood walls. I did black plastic the first year and in 30 min half the haunt was down so we had to stop and "repair" grrr. I went to tarps the next year they held up much better. I found some black on one side and grey (silver) on the other side at Northern Tools on sale that year.
I have no problem with using black plastic here and there as a fill in, but a whole haunt made from it, and not sprayed with fire retardent or made from fire retardent plastic, is a nightmare waiting to happen. And fire retardent black plastic is not cheap, and only has a shelf life of 6 months or so. So long term, it's not a cheap solution, and besides, it looks like black plastic, not walls. Cardboard, fabrics, and foam can be painted to look like walls, but there is the ever present danger of someone pushing through, or damaging them in a moment of fear. I have also gone the route of making 4x8 foot wood panel sections for my walls. >>>>>>>>>>>>>> http://terroronbeechwood.com/index.php?pr=20052
The problem sometimes with cheap, is that it is rarely a permanent solution, and if you have to keep paying for "cheap" year after year, it adds up.