Getting a 2'X4' in that budget is going to be tough unless as mentioned you kit build one yourself. If your not into a home built, I suggest the probotix Fireball comet with a 25" x 25" cutting area. It's a ready to cut setup. I use probotix components in my PhlatPrinter and they are rock solid. I have heard several kit builders make the comment that by the time they finished there kits, they figure they could have bought a machine that was already complete. Don't know how true that is but it doesn't surprise me.
2'X4' is overkill for cutting formers and spars. You may even be able to cut it down to 12" X 18" and get the Probotix V90 for Under $1400.
http://probotix.com/FireBall_Comet_cnc_router/