Hardlinks - be it adult or mainstream - is an example of where the market tends to price itself ... with one of the main contributing factors being good ol' supply and demand ...
So the most accurate answer actually is
"whatever you are able to get for them".
I sell mainstream hardlinks across my network of 26 blogs ... and I have seen market value dictate my prices between $40 and $60 per link one month, and then 3 months later, I would be hard pressed to get $25 per link ...