I bought a used bike off of FB marketplace for $40 (ETA: and that was in rural Newfoundland where no one ever gives things away for free, I'm sure I could get one for free here in the city, or much less than $40)
If you don't need anything fancy, literally any bike will do. Granted, if you are larger, you might need something with a higher weight tolerance than a lot of cheaper bikes can handle. I'm a small woman, so even the cheapest bikes are sturdy enough, but my old, 6'4" roommate had a nasty incident with a cheap elliptical once.