If by "mechanically good" you mean it times properly and smoothly on each chamber and is within spec for forcing cone-barrel gap, headspace, etc. -- that with your cosmetic description puts the 10-8 approximately in the NRA Modern Gun Condition Standards "Good" grade. Puts it in the $250 to $300 range.
Pictures might change that up or down depending, as would mechanical checks. If pictures show it's less worn cosmetically than one might imagine from description, and the mechanics and spec checks turn out to be cherry, value increases. If pics show it looks like its been dragged behind a car and barrel-cylinder gap checks at .014" and she spits lead, value decreases.