Lucy,
If you're looking for a round number for swapping out the finder check with DAG camera repair. He's Leica trained & has a bunch of used parts perhaps he could get you some numbers.
Keep in mind cheap is relative if you have to spend around a grand to get another body. If your (camera)body works well & you like it except for the finder, it's an argument against buying the unknown quantity of a different body.
I'm stretching the memory now but don't you lose the 35mm field when changing to the .85 finder. No big deal if you're not going to use a 35 though.
Just read the OP again, aren't the MP's few & far between on the used market?