80-90% depending on how popular it is. The old 3-9x40 Rifleman probably 80%. The newer Mark whatever fancy mil-dot IR models probably closer to 90% or more.
I think the funky VX-L models with the crescent shaped housing are the only ones that would fall below that number. They are kinda odd.
All in all though probably one of the better optic investments. The quality and warranty help them retain value.
The best answer is it depends. There's a sucker born every minute, and the world is also full of low ballers.
Generally, I won't pay more than 75% or new retail for something like that. The 25% hit comes from the fact that I don't know what you did with it, plus I can't send it back if I just don't like it.
The fact that Leupold offers a lifetime warranty, transferable automatically, is a great selling point.