servo on the basis

I want to build a servo on the basis of a windscreenwiper motor from a car. The idea is that it should be opreated by a regular RC from RC car or boat. Is this possible and can it be rather cheap

Yes, it's possible, you need to steal the insides out of a normal servo. Then you add a potentiometer to the output of your wiper motor and connect it to the feedback input of your servo board. Lastly you need to have a MUCH larger H-Bridge between the servo output and the motor.

I used to have a link to a petrol driven robot that used a wiper motor servo for steering, but I can't find it now. Here's a link with a picture of one driving a baseball bat though!