If you're on a time and materials budget, I've found that blue tarp from Home Depot or Cambodia Tire are great for walls. Its very light weight and easy to cut. Use rope/twine and tuck tape to secure the tarp to hooks in the ceiling (assuming you're using an unfinished basement) and use 2x4s to hold the tarp down straight by trapping the bottom of the tarp between the floor and the 2x4. You can use 2x4s to make strong door frames that are suitable for button hooks and banging into when doing entries. Same goes for corners.
I have a video walkthrough of the one we built in my unfinished basement (being uploaded). We were able to build a two room set with its own lighting (scored from the takedown of a movie set at Downsview) and we even wired the place with 4 cameras and hooked them into a survellance computer setup so we could monitor the action remotely upstairs and record it. I've got gigs of wolves screwing up their entries and one of me kicking my P14 down the hallway (accidently) as it came loose from my crossdraw).
The basic cost of the tarp materials was around $200 (we used a LOT) and about $100 in 2x4s. The cameras were ones borrowed from teammates which had video outputs. I ran the wiring through the vent system to get it upstairs. It took about 4 weeks to complete.
So, it can be done on a budget. Oh yeah and one other thing, tarps enhance the experience because of the report when BBs hit it - you get four guys entering a room doing auto bursts, its awesome sounding.
Also note the standup targets we built, which can be placed anywhere in the room and be used in lieu of an opfor.