I've usually purchased based upon what another board member or someone in my local has they're looking to sell. Luckily most of the glass has been (for me) higher end. Or something cool, like an old redfield 4x, that came with a 77/22 package from Ruger way back.
If you're going to play the LD game, save the pennies. The optics game is like AR acc. Someone, somewhere, wants to "upgrade" and if you act fast usually score something worthwhile.