You can buy cheap servo motors from ebay for a price of less than a gear motor. But these servo motors can't be used for continuous motion. A simple modification can be done by removing the locking mechanism as in the below photos. I'm using a metal geared servo motor from ebay for this. You should also glue the potentiometer in the middle postion after cutting the lock in the gear. You can use different pulses with varying pulse width and frequency to get continuous motion in the clock wise and anti-clock wise directions.