First thing that comes to mind is an adult mobile site.
Domains in the adult domain aftermarket isn't too hot unless it's an aged, Premium .COM, these guys know a lot about domains, will know off of they bat you got it for regfee, and they take that into consideration. plus it's hard to contact endusers since most of them keep their information private, and they usually won't touch anything that isn't .COM
But, I'd say if you found someone serious about this niche that has a mobile paysite that does well, they'd offer $50-$150 for it. Beyond that would require a lot of luck.
IMHO