i've been looking, and i don't see anything cheap when it comes to PDW. if you want to shrink the buffer tube you get into proprietary parts, and that's money. Add a special BCG for some with their fancy coatings, and you're into $400.
I ran an NEA for a little bit, but it was heavy and I pretty much hated it. I'm still looking for solutions, as I have a 7" 300BO upper now and I would love to get the shortest overall gun imaginable, but I'm not willing to pay $$$ for it, not worth it for me.
I may go with a Battle Arms mini stock thingy just to throw on the buffer tube, but I'm undecided at the moment. I played with an LWRC kit and that was nice, so it's an option.