The best method to price a lens is by seeing what the market value of the same used lens are. There are many different ways to determine the value across different platform. For each of these compare like for like, so as well as condition if you have a box/filter/hood/case/cds/leads/remaining warranty etc compare sold values to lenses with box/filter/hood/case/leads/remaining warranty etc.

1. eBay. Probably the easiest way to check search for your lens used and finished bid. BUT eBay takes a cut.
2. Classified adds website (e.g. Craiglist/gumtree)
3. Camera shops selling used equipment. Definitly take these with a pinch of salt. They like a bit of a markup (saw a lens over the weekend the same as one I sold six months ago. It had a 40% price increase) but may come with a warranty/returns private selling won't allow.
4. Get a value from a dealer selling used gear. One web based one in the UK (MPB photographic. (No affiliation)) gives a rough estimate based on information provided and pictures given, but a more detailed estimate if you send to them. They will sell these lenses on so that is how much they'll buy it for. Note not how much they will sell the lens for HOWEVER, the one time I have used one of these services, the amount they offered me was the same as I what I privately sold it for.
5. Commission sale. Some used gear dealers will sell equipment based on commission, so they'll give you a value and then make a commission on selling it on your behalf.
6. Facebook groups. Checkout local photographic facebook groups to your geographical area, many of these allow the selling of gear (some devoted). They're all private sales so no commission or increase.

One thing to remember with selling, there tends to be two options of selling. Selling for the ideal price. Or selling for a quick sale. Quick sales tend to be cheaper because the current owner wants the cash, where as selling for the price you want for it may take some time to shift it. If you want a quick sale, cash and none of the hassle. A user gear dealer is a good option.

There's also nothing wrong with pricing a lens at how much you think it's worth. Even though it's above the market value. Someone may pay that for it. There's nothing worse then selling something and later regretting it as you could have got more for it.