Dan
First the 90/2.8 EM is an excellent lens that I use frequently. The M version of the lens (latest ) has the pull out shade and uses a 46mm filter . The used price can vary considerably based on the age and condition of the specific lens. A mint 6 bit coded lens in the silver box will be considered the current model and will sell for over $2000 and can get up over $2500 if its like new. A used older model (white box) without 6 bit and in used condition can still run near $1500 . Occasionally you can find a used lens for a few hundred less.
The prices have increased significantly over the past two years …the good news is that the prices look to hold up well . I found a nice used one and sent it to DAG for 6 bit coding and calibration ..now its one of my favorite lenses.