What ideas for finishing concret floors for a very small budget?

Depends on where this is located. If it is outside and you are concerned about traction just brush the concrete with a shop broom. If this is a patio maybe a nice stamped effect so it looks like random stones. You could always throw some dye into the wet concrete so the finished product is tinted instead of just plain gray. The stamping forms or dying concrete should both run under $100 minus any extras of course.

Floors in what area? Outside?

Floor Paint - Paper Bag Varnished finish

We painted a concrete floor, but the paint flaked. If I could do it again, I would get concrete stain. It sinks in and shouldn't flake.

Paper bag flooring. I love mine! You just get a roll of contractor paper at Lowe's or Home Depot for around $8 and get a can of varnish. Make sure your varnish is for floors and not oil-based. If you wish your paper floor to be a different color you can also stain it before you varnish it.

Epoxy is more durable then paint on a concrete floor. You have to work faster with epoxy then paint and it is thicker but more durable. A throw rug would give you some cushion and be warmer to walk on. Also could take it outside to shake out the dirt.

Thankyou