I would say that if they are asking $3000 the car must have some serious problems - like maybe it doesn't run. A 420SEL that won't run is worth about that much for salvage. Have you seen the car? I love my 420SEL. I can't compare repair costs as it is the only MB I have owned, but certain things can be expensive. If the car is well maintained and has never been in an accident (no body damage), rust free, etc, it should be worth at least twice that much regardless of mileage. I would be pretty suspicious unless this is a forced sale (death or divorce).
1995 E420 - retired
1986 420SEL - retired