Update

We finally completed our robot, after buying the missing Lego parts:
Total: US $0.18 = $0.06 ea.
Shipping: US $2.50
Insurance: US $0.00
Additional Charges 1: US $0.00
Additional Charges 2: US $0.00
Credit: US $0.00
Grand Total: US $2.68

                                                              Before:

After:

Robot height: around 8 inches

Sensor center height: 7 inches

Width wheel to wheel: 5.5 inches

Thanks to Patricia for directing us to bricklink.com

Thanks to Joes Brick Depot for their cheap lego parts. Onwards!