Lots of good advice here. Bottom line is, each case is different. Rental fees tend to cover consumables like tires, brakes, etc. Mechanical failures can be tough. If you miss a shift, and zing the motor, that should be on you. If it's a fresh motor, and was put together poorly, and lunches, it shouldn't be on the driver(s).

Split driving duties, like enduros, can be done a couple of ways. Either the driver behind the wheel at the time of the incident, is on the hook for the whole thing, or you can make a prior arrangement when all the drivers split the cost. I was involved in a 24 hour race, where there were several drivers in the car. The deal was, that the damage would be divided by 1 more than the number of drivers. Everybody was on the hook for what ever that amount worked out to be, and the driver was on the hook for the 'extra share'.

It all depends on what works for you and the car owner. Most importantly though, get it in writing, up front. This includes an agreed upon ball-up price for the car, and who gets to keep the wreck. My take is, if you wad the car up, and have to pay for it, you get to keep what's left (you essentially bought the car). If you can recoup some of your expense by parting out what's left, that's your perogative.