It's always better to use the real thing if you can afford it. However, when you're on a micro-budget anything that you can do will be a help. Moving blankets, as SinEater mentioned, are a heck of a lot better than nothing. Even comforters and quilts can assist in mitigating acoustic bounce. Carpeting on the floor is another big assist. Don't forget to put carpeting under the crew; it's amazing how much noise even a disciplined crew can make. My studio is lined with Celotex fiberboard, and I used the leftover sheets to make baffles and gobos. Celotex, depending upon the amount of sound absorbtion, starts at about $30 per 4' x 8' sheet.
What you want is to diffuse the sound, so when hanging from C-stands (or whatever) don't stretch the sound blankets (or whatever you use to substitute) tight; leave lots of folds. You want to create lots of angles; a room is lots of parallel surfaces which exaggerate frequencies and create standing sound waves. Keep everything as far from the walls as room allows.
Insist that your crew and try to have your cast wear soft-soled shoes. It is common practice, working in conjunction with wardrobe, to put felt or rubber on the soles of footwear that normally has a hard heel/sole; the hard sound is replaced in audio post. Don't forget to sweep hard floors; pebbles and other grit are greatly exaggerated when recorded.
BTW, FreeCycle is a great resource. I've found lots of carpeting, drapes and fabrics, and I've gotten quarter and half full boxes of ceiling tiles. Once I even got a box of real Auralex sound-proofing worth almost $500. It doesn't have to be pretty, it just has to work.