Replacement cost will be a little less than $200.
I'm not sure what your problem is, though. The mirror hangs off a plastic "foot" attached to the roof of the car. Is this piece shaking too, or is it just the mirror itself?
If you take hold of the "foot" and yank (not the mirror itself) you can remove it and try re-attaching (this is quite difficult and needs a bit of brute force). It may be that you have knocked it part out or something.
1999 SL 500
1993 320 TE
1986 500 SEC