From my experience of currently working in a tool rental place.................. Your going to need a really solid agreement contract, like have a lawyer make it up for you. Next, forget the online idea, its hard enough to get people to return them when its in person and they live down the road. Next, you will need multiple camera's. Probably 10 if thats the only thing your renting. If your only charging 20$ a day and you have to pay for the camera, gas to get to the track, and a whole day of your time your not gonna make much money. Plus you will always have broken ones. And say its a muddy slippery day and there is a pile up. 3 of your camera's break at the same time. Now you have to exchange them for 3 other camera's so you now have 3 broken 300$ go pros and 3 out, so you have 4 left and you've made a total of 60$.
Bottom line, i think to make any money you will need to go at it at a full business approach and get 10+ cameras and have on-site video editing and a way to put it onto disc/digital media, because that is where you would make the money, not the cameras themselves.
Sorry for the novel, just telling you from a rental guy