I've been toying with the idea of putting a German optic on my new (to me) PTR91. $599 seems awfully high even if it's graded 'Excellent - Like New Cosmetics and Function'

http://www.robertrtg.com/z24optickit.html

They also have the ZF Model 1 for $429. Are these optics GTG or are there better options? Anyone know of a reputable vendor that would have them for cheaper? I don't need pristine cosmetic condition but I do what it to be 100% mechanically sound.