I really don't believe that this rifle to scope cost ratio is valid. The biggest reason is that I've seen plenty of sub $500 rifles shoot sub MOA with good ammunition; like the Remington SPS models and the Steven 200 models. On the other hand, when it comes to scopes, you usually get what you pay for.
It's already been said many times in this thread, don't buy according to a ratio, buy what you want/need. There are many decent scopes out there, but it's a real matter of what you need it to do.
You can get a crap scope for under $100. It will allow you to see through it and zero it in (hopefully). You could spend around $300, get some decent glass, but you're not going to get the resolution that you will get with better glass and you are going to see a good bit of distortion, especially under recoil. You will also have to worry about how well it will fare in a fall. You could spend around $600 and start moving into some good glass that has good resolution and low distortion, but how repeatable are the settings and how robust is the build. You could spend $1000 and get very good glass with very good resolution and very low distortion. At this price point, you'll likely get turrets with very repeatable settings as well. Now, spend $1500 and you will have all of the above, plus the robust build. Spend more and you get more, although the law of diminishing returns will strike.
Personally, the Nightforce line is a favorite of mine, because at that price point I can get all that is important to me. I like good glass. I don't need the best, but it has to be good. I like good resolution with a sharp image and don't like to see distortion under recoil. I like being able to see my hits. I also like the fact that I don't have to worry about losing my zero because one of my rifles got banged around.
As has been said, it's all about what you want/need; but to say that you should only spend X on a scope because the rifle only cost Y is foolish.