Sub $15 DC Motor and budget wheel recommendations for college project

Our robot so far:
-Will not weigh more than 20lb and in fact, may weigh even less.
-Doesn't need speed. It just has to roll when told to do so.
-This is for a freshman engineering design project that is really just an introduction and not serious robotics or engineering.

What we are looking for:
-Sub $15 DC Motors
-Cheap wheels of preferably 4in diameter and up.

Recommendations preferred but advice along with recommendations is appreciated.

What is your opinion on these geared motors?

From my experience your not going to be able to push a 20lb robot with dollar store motors. This link might help you find some good motors.

http://www.robotmarketplace.com/products/motors_main.html

How about a gear motor with an integrated wheel?:

They might be a tad slow for your needs (especially @ 12 VDC) - but as someone who has bought a few in the past, the "wheels" are fairly rugged (at least for an indoor, non-carpet rover), and the torque is really good, even at 12 VDC.