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. >>>>>>>>>>>>>>
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.