It’s worth remembering a lot of the earlier FSD purchases were at much lower prices. It’s been as low as £1k ($1.5k) over the price of EAP, or about £4k in total, so half the price it now is, plus those cars may be due hardware upgrades. The original owner also had the use (albeit mainly just EAP features) for several years. If you look at the subscription model cost, 3 years of use is about $3k, the seller might also get $2k more than not having FSD when they sell, so they’ve had about $5k of “benefit” for their $5k outlay. Seems fair.
If you think as an option it’s expensive, what extra benefit do you really get for spending $4k on bigger alloys, $1500 on different colour seats etc when you come to sell? You’re lucky if you get 1/10th if that when you sell.