Are you looking for one for archery or rifle? I never saw a mechanical one but I would be interested in how they work.
Back when I had a lot of time to spend on hunting, I was pretty good at judging archery distance. I have gotten worse and the bows have gotten better (longer shots possible) so I broke down and bought one for archery hunting last year. I think it was around $150 and it works pretty well at archery distances but the display is sometimes hard to use.
What I always did in years past was sight to paces instead of yards. (For me, they turned out to be fairly close). When I got to my stand, I would walk off to a couple landmarks like a boulder or a tree and remember the distance. That way you could use them as a guide in estimating range.
Now, if you are talking about a rifle, I have no idea. I just ordered a range finder this morning so I can try to improve my accuracy at distance. I ordered the Sig Kilo 200 for $350 shipped. That seems to be a pretty good deal and they are running a special right now (normally $499.). My guess is that they are clearing inventory for a new model.
