Check Reverb, see what other people are asking for the same thing, decide where in the range you want to be based on condition and what you want and how long you're willing to wait for a sale, and go from there.
The market is all over the place. BKPs sell for stupid money. Sometimes as much as new. Duncans usually sell for about 2/3 of new. Fishmans have dropped a lot in the past few months. Etc etc.
Depends on manufacturer and model, but for the regular Duncan production line, I typically see used for $65-$79 maybe, while they are $99 new. That's about 20% off or more. Depends on condition also, is it bone stock, or was it messed with, etc.
As mentioned, brand is a big determinant..But even in a common brand, if the pickup is rare it can bring more than a garden variety one.
Also though, with a particular model that isn't widely-known, it may take a while to move.