I don't believe that there is any rule of thumb on depreciation. There are a lot of variables that go into establishing a value. Condition, accessories, miles, gas, electric, type of enclosure, etc. For instance you can find a 2013 gas yamaha anywhere from 5900 up to 7000 or 8000. You really have to shop around and look at the many sources that list golf carts for sale to get a real feel for cart prices