My preferred unit is the Okna 800dx, as well as the Soft Lite Imp LS, or Sunrise Vanguard.
Why do you like the 800dx over the 500dx from Okna? What pricing delta would you expect to see when upgrading to the 800dx without adding any high end features and using a standard size UI?
Last question why do you put Sunrise in the same category as the other 2? While it is a good window, it does not have some of the additional structural upgrades to the frame or sash as the other 2.
800 series has the reinforcement rail, magnetic weatherstripping, and an applied metal handle.
DP is a bit higher in the 800 at a DP60 but the DP50 is fine for the Okna 500 or any other window that is near that number.
What someone charges for the upgrade is up the company. Our upgrade feel from the 500 to the 800 is very small.