Assuming you find an inexpensive SS rear wheel (cheap 5sp 26" wheels exist, cheap SS wheels are harder to find), a couple cheap BMX sidepulls, and a freewheel, you're looking to spend at least $50.

If you have access to a bike junkyard, this project may be doable. But the lowly coaster brake truly is the solution to lots of problems. If you must have a cheap ss freewheel bike, go to walmart, get the cheapest MTB, park it on one gear and ride it.