Robot Wheels

I am looking for 2 geared motors with a standard shaft size. The motors will be used with a caster wheel. I will also need mounting holes for the motor.
The motors should be close to 60 RPM and be able to hold about 15 pounds.
(if they have wheels already attached thats fine too)

Now, I will need these motors to be cheap although it's ok if they are a bit expensive (<150 US $)

Can anyone recommend any?