which motor to buy?

I'm getting into robotics and am a bit low on funds. I need an alternative for a servo (expensive :confused: ), a dc geared motor fits right in the budget. So basically I've done the platform out of cardboard and I am looking forward to a 2 wheel drive chassis. I shortlisted a few motors (link below). I need help finding a right one. Thanks :)

http://www.amazon.in/dc-motor/s?ie=UTF8&page=1&rh=i%3Aaps%2Ck%3Adc%20motor

The "right one" depends on the mechanical characteristics of the robot. There are lots of tutorials, like this one.

However, if you are just looking for cheap motors and a chassis, get a discarded toy from a thrift shop.

I would not use the ones with 1000RPM. They are too fast. 100RPM sounds reasonable for a robotic car. How are the prices of amazon compare with ebay (some Chinese sellers are always selling cheap electronics)?