Hard to say, that depends on your negotiation skill and the vanity address itself. I never see anyone attempt to sell vanity address with such length.

But if you don't mind some prediction with regression. Then with with data from bitcoinvanitygen.com and http://www.xuru.org/rt/TOC.asp (for regression tools) :

• 7 char prefix - 0.0039 BTC
• 8 char prefix - 0.0179 BTC
• 9 char prefix - 0.0499 BTC

With linear regression we'd get y = 0.023 x - 0.1601 or 0.2309 BTC
With polynomial regression (degree 2), we'd get y = 0.009 x^2 - 0.121 x + 0.4099 or 0.9539 BTC

But since you already know the private key, then the value of the vanity address for others is 0BTC.