On road motors have more ports for higher RPM, off road motors have less ports and are designed for torque.
There are a few "economy" on-road 21's out there that wont break the bank, and would be good for learning, but the performance will lack in comparrison to the top motors (obviously)
Here are a few:
http://www3.towerhobbies.com/cgi-bin...&I=LXRYV2&P=SM
http://www3.towerhobbies.com/cgi-bin...&I=LXXKX8&P=SM