are these light enough that those cheap $40 RC helicopter can carry it?
No way. Too heavy.
Cheapest way is to get an 808 keychain camera ($8-40 depending on version). They can be carried on quadcopters like the V929 (and larger).
The V959 comes with a camera pre-installed. It may be able to be triggered from the remote. (A similar version from MJX can be).
Hobby King has a wing camera for $30-40 that I just ordered. The quality is higher (HD, 5mp), and it supposedly has motion sensing activation. It's probably the closest thing you can get to GoPro quality, without needing an expensive quadcopter.