With scopes, it can be a bit tricky because there are not any easy and clear cut measurements of quality. The biggest thing you will be paying for is quality of the lenses and their coatings. Because MOST scopes are built from just a few manufacturers, (and in turn rebranded) the level of optical quality is a direct relation to the price of the scope. i.e. you generally get what you pay for. Buy the best you can afford. One think you can look for is light transmission, but that measurement is not cut and dry either. Also, better optical lenses and coatings more then makes up for bigger objective lenses. a 50mm Objective will not make up for poor lenses and is used as a selling gimic. Just pay for good glass, and you'll be happier.
Mel is certainly correct in his evaluation of scope quality. But what I have learned to look for in a scope, as a way to tell if it will be a good one or not is if it has that golden ring around the objective bell, telling you that it is a Leupold!
The mid-range scopes from Zeiss are of very good quality too, and the quality of the optics is _very_ good(There's a reason why they've had a reputation for high-quality optics in scopes, binoculars, oculars for microscopes, telescopes, camera lenses etc for over 60 years)
Hensoldt scopes are of superb quality, though you've got to pay for it. Nowadays Zeiss owns Hensoldt, but it's retained as a separate brand, with their own optics specialists and designers.
Unfortunately, I have no experience with Kahles scopes.
If you can afford it, Schmidt und Bender scopes are also of very high quality.
I am a very big Leupold advocate. For many reasons I prefer Leupold over other scopes. A couple of them are that the scopes are of high quality and Leupold does as much as they can for their costumers. And a big bonous when I'm buying it is that they are all made in America. But thats just what I think.