I believe the valve that needs to be replaced is the "mono valve", and I'm not sure if $900 is the proper going rate, but I do know that the valve is very cheap, and the price of the job will be almost all labor.
The dash has to come out, and thus very labor intensive. For example, the evaporators sometimes go out on these cars, and it is buried under the dash, and the job is 22.5 hours shop labor.
If you're going to go through the trouble to do this, there are other things you ought to replace when you get in there, like the vacuum elements for both the center vent and the recirc air one, too.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".