Wow! This is a hard car to put a number on with the mileage so low. Since I have a brother that is in the car business I have access to a website that shows all recent sales data for all dealer-only autions accross the country. I'm not sure where you live, but if I use the national average, which includes sales from all auction houses, the average sale price (dealer wholesale) is $13,552 with average mileage of 96,296. This average is based off of 30 sales with a high price of $18,000 w/ 70,549 miles at a Florida auction to a low price of $8,000 w/ $150,000 miles. Typically, each make/model is adjusted by different rates per 1,000 miles of change in actual miles. For this car I would use $125 per 1,000 miles for adjustment, resulting in a total adjustment of $10,125 {[(96,000-15,000)/1000]*125}. Add the average price of $13,552 and a good approximation would be $23,677. Now, this method is used for cars with changes in acutal miles to average miles that are much closer together - maybe 15 to 20 thousand miles. When your looking at a difference of 81,000 miles this method may not be sufficient.
Two sales of interest with relatively low milage were as follows:
Black S420 38,836 miles $15,100 in Chicago
Silver S420 47,000 miles $17,800 in Bordentown, NJ.
Let me know if you have any additional questions. I don't know what the condition of the car is, but I would be careful about buying a car with such low milage. My reason is that the car has been sitting for most of its life and once you start driving it regularly, things typically begin to break due to the increased usage - not just with w140s but all cars. Also, since the car belongs to a friend of yours, I'm sure he will give you a good deal.
Good luck and keep us posted.
