It depends on the bike. I buy, refurbish & sell bikes for hobby money. A Walmart or Target quality bike is not worth buying and fixing. Even if it's free sometimes it's not worth picking up. I found a Nishiki hybrid in a dumpster that should have stayed there.
If you can find a decent quality bike that doesn't need much beyond tires & tubes, go for it. A lot depends on how much knowledge & tools you have on hand. My current favorite bike was a Univega touring bike. I bought it for $40.00, spent about $250 fixing it up with top quality stuff. It was worth it to me. I had it powder coated for $115.00. Painting is not usually cost-effective or necessary.
Post pics. Check out the bike market in your area. In my area (Southern California) new bikes can be a better bargain than used.