Brian, A friend has a 280Sl with similar mileage. Sweet little cars.
Similar to the 380sl with a smaller engine - very well balanced for a panzerwagen....but the American mentality is bigger is better and the 280s seem to falter on the pricing. If you look at the price guides you should see the 280sl with similar pricing as the 380sl. They are nicer to drive than the one with the V-8 lump but guys I talked to incorrectly want the V-8.
I have been watching pricing starting several years ago as I didn't think I would ever get a 113. A beater should be in the $5-6,000 range and go up from there. I haven't seen any higher than $15,000 but I have a very limited inventory to guage my opinion on. I had a look at several 380-450-560 and 2 280sls and both of the 280s were owned by real enthusiasts and were in excellent shape as compared to the 450 and 380s which were run hard and put away wet.
Check the german or British magazines about the 280s - you may need to educate the buying public when you help sell the car.
Does it have a 5 spd or automatic?