I would guess CFV-II is a bit more expensive than other 16 megapixel backs like the H-mount P20, because it is a Hasselblad and fits and looks good on the V-system. Otherwise 16 megapixel square sensor backs are not that expensive. Here in Sweden I now and then see H-mount P20 for about $2400 on the second hand market. Searching the "for Sale" forum here on Lula for recent sales can be a good indication of both availability and pricing.
Getting the less-good-looking but 4:3 22 megapixel Hasselblad CF22 could actually be cheaper than a CFV-16. Unfortunately V-mount Hasseblad backs are not that common on the second hand market. I'm guessing many keep theirs in the closet or just for fun as the value now is so low that there's no big money to gain from selling it.
About the e-bay example, I would not pay that amount for a 16 megapixel back, for $6700 you should get say a 33 megapixel Aptus 75, and possibly a 39 megapixel CFV-39 (also rare on the market). I guess the seller is hoping for someone that desperately wants a V-mount back.
If I would myself buy a 16 megapixel back today, I'd think $2400 would be the "right price", and possibly I could stretch to $3000 but I would not pay more.