Motors do look cheaper from the unexperienced point of view but...
As mentioned above you will need to buy additional circuitry to use a motor. motor controller / hbridge (for each motor).
Gears. A motor normally wont have enough torque to pull a robot but they spin at 10,000+ rpm (think about how fast 10,000 revolutions per minute is... Ive got one that spins at 29,000 rpm that im gonna use to mod an r/c helicopter with). You have to gear down the speed and gain torque in the process, reducing speed with gears gains pulling power. So you have to include a gearbox for each motor. Or buy motors with gearheads attached(probably more expensive than a servo).
Youll soon see that servo's are generally cheaper (a servo is basically a device that includes a motor, a gearbox and a h-bridge built in - with its own feedback system and control system too).
The difference is that with servo's you are limited to the speed and torque of the servo. Building your own drive system you can specify your own speeds and torques to suit your requirements (but at a cost)