There are some formulas used by the people who buy and sell used truck mounts like me, but here are some guidelines to consider:
1. How many hours are on the engine? And if its an air cooled or liquid cooled engine?
2. Is the manufacturer of the machine still in business?
3. If the manufacturer is still in business, is it a current production model, or one that's been discontinued?
4. What is the physical condition of the machine? Especially the recovery tank?
5. Was the machine ever frozen?
6. Do you have reliable maintenace records?
7. Even if the machine is made by an existing manufacturer, and even if its a current model, is it a popular "in demand" model, or one that isn't well known or well thought of?
8. Most importantly: Whether you own it, or are looking to buy it, think of this: You can end up depreciating the value of the thing down to 500.00, but if it can clean carpet reliably, then the "value" of 500.00 is meaningless. Its value to you as a machine that can reliably produce income versus buying a new one for 15000-30000 dollars is far higher than its depreciated value.
Disregard all of the above if you were just asking for tax reasons.