High Power Optics
Lots of useful info in reply to your question on popular high power optics. You didn't mention what type of rifle you are using or, if you're loading your own, what your're pushing. When shooters start reaching out to 600 yds and beyond, they're usually not shooting factory ammo (unless it is produced in the Army Marksmanship Unit "Factory").
It's tough to give you an objecitve answer with so many unknowns.
If you are using a factory produced rifle you should expect to pay almost as much for your glass as you did for your gun. My personal favorites are made by Nikon...Monarch Line. Outstanding quality and a lifetime guarentee.