IMHO , as one who uses a crossbow for most of my bow hunting these days, any scope you can see through and will hold it's zero will do. You are talking about using scopes far more accurate than the weapon they are attached to.
Average crossbow shot is likely less than 30-50yards. True, some high dollar crossbows will do 100yds, I hear, but I would never take a shot that far with a weapon with that many variables.
I have shot targets with a longbow, recurve, and compound out to 70yards. (Measured course) but I will not take a bow shot at a live animal at more than 40-45yds max.
Even with carbon shafts, and new matched razorheads, pristine fletchings, and perfect form , variations in flight will still occur, due to factors such as the shift of the breeze.
Scopes are also susceptible to the problem of parilax. I buried a crossbow bolt in a thumb sized branch between me and my deer. That branch was not visible in the scope but was in the flight path of the bolt. Cost me a nice buck. 4X Scope was removed after that, and only one pin is used now.
A high dollar scope on a crossbow is like a precision guidance system on a wiffle bat.
