Like most any hobby, you can spend as much on it as you like. But the bike coop in your city will probably sell a perfectly-serviceable used commuter to you on a need-based sliding scale all the way down to $20, if not totally free.

As for legs... They're nice, but I've ridden my bicycle 110+ miles in like 8 hours. That's a massive quantitative difference, and a huge impact on quality of life for someone who needs to get around.