If its them model that they sell at Autozone, Advance Auto, etc, they're like 100 bucks new. If it's in good shape and what not, maybe 50-60 bucks. Try a little lower first, you might get lucky.
I like having my after market tach in my GP. It keeps everything at eye level and with the shift light, I don't have to keep looking down to keep from hitting the rev limiter. I can tell by the sound of my car, but I still like having it. To each, his own. Some will like it and some will give you crap for it.