If this is for a non-FRC project, then you don't need to adhere to typical allowed motors.
Cost is obviously important for you, I would suggest looking at the gearboxes and motors sold by BaneBots, their gearboxes and motors are on sale at the moment, same with their wheels.
http://www.banebots.com/category/P60_7.html
The motors you select are dependent on the weight and size of your robot.
But a 550 or 775 motor should easily be fine, for a driving a decently sized robot
http://www.banebots.com/product/M5-RS550-12.html
http://www.banebots.com/product/M7-RS775-18.html
For wheels have a look at these
http://www.banebots.com/category/COLSON_WHEELS.html and these
http://www.banebots.com/category/COLSON_HUBS.html
With a robots project, the motor controllers are going to be a big cost component and would probably be about $45 if you went for the low cost REV Spark