OK, I commute with my bike every day and I kinda would like to know how far I'm going and more importantly how fast. I don't need anything difficult cause I'm just curious about stuff like that for fun.

I ride a Kona Smoke 2-9. Any suggestions on what kind of device I should get? I don't want to spend a lot because I'm a poor student so I would like to get something like $20 and below.