You need the trip computer display unit, the rotary, panel switch and the short wiring harness that goes between the two.

$50 is reasonable, but you can get it cheaper at the local pick'n'pull. Most 850s came with these trip computers and almost every car at the pick'n'pull has them. I converted my car several years ago for less than $20, using parts from the pick'n'pull.

I believe there were two versions of the units corresponding to the two different speedometer units available on the 850. Make sure you get the unit that is appropriate to your speedometer unit.