Hard to put a price on it. The right buyer who knows exactly what to do with this domain name would pay thousands. If you can afford to hold on to it, do so. XXX is synonymous with sex. I predict this extension will take off one day. The websites that felt pressured to buy their domain with .xxx to protect their brands weren't to fond of it... but I see this TLD being the top extension for the sex industry soon.
I have to disagree with you, Many porn sites have shown they are against the .xxx extension. I believe that the firm behind YouPorn has filed an anti-trust law suit against ICM. They don't want to be associated with .xxx. Makes them easier to block.