Perhaps you could get a new lock at a junkyard and then take it it a locksmith to have it made to fit your key.
This should cost less than $150, but I'd call the locksmith first. Mercedes keys and locks are not standard after 1990.
There is certainly nothing in a doorlock that should cost that much.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"