I don't think they are expensive, I paid less then $100 for the one I put on my Ruger Mk III 22/45.
It works quite will. The do have to be adjusted, but its easy to do. They work well in low light conditions and work even in bright conditions.
Well worth the price and effort in my opinion.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071