I think most of the cost in quadcopters are in the sensors. I looked for cheap arduino copters and found this http://code.google.com/p/quaduino-ng/wiki/ShoppingList

That's silly. You can buy a complete arduino based flight controller with 3-axis gyro and accelerometer for less than $13. (For the best value I'd recommend the KK2.0 for $29, and for more comprehensive sensors one of the APM based boards, but you get the point.)

Another big cost is replacing and fixing the ones you crash.

Not quite, this is probably as close as you're going to get:

http://www.youtube.com/watch?v=XYi2NrEAxrs

Hubsan X4 £35-£40, Camera, TX,RX £20-30, Monitor, varies.

I suspect power would be an issue. You probably want to dedicate as much battery power as possible to the motors.