What is the value of a Harrington and Richardson topper model 158 12 GA?

It varies with condition, which is probably most important factor. But you'll find nice examples for around $75 - $125 at gun shows. I recently paid $80 for mine - it has the original recoil pad, nice wood finish, and no rust. These shotguns seem to be (like most H & R guns) bought to be used and not really prized by collectors. If you're going to use it, do so without regard to it's long term value. If you're thinking it's going to be worth lots of money someday- don't count on it. As long as you take care of it you'll never lose any money on any H & R 12 gauge.