Find the price of "competing names", ones that would make a fair alternate for your domain. Check BuyDomains/Afternic, Sedo, etc.
Absent type-in traffic, which tends to demonstrate the embedded/persistent strengths/value of a generic domain, it's hard to establish a baseline price. IF it was receiving traffic then you could look at that element of value from the "converted leads model" of analysis we've talked about here, i.e., enduser value.
I know of no automated valuation service that nails value.
You can post domains "for appraisal" at various domain forums but the process is very hit-or-miss. Still, it's free and, if the old warhorses get involved, you can get a better sense of value. They only tend to join the discussion if the domain has some real ($10K-100K+) value.
When in doubt, aim high . . and negotiate.
Lastly, the market for s e x related domains has tanked. There's so much free . . food for the imagination . . that's available that paying for apparently is unnecessary, killing the profits.