I don't think there is a generic calculation. If your site is built of non-exclusive content and each month the member base is dwindling then it would be hard to ask 9-12 months of your current revenue.
However, if regardless of the member base your paysites still receive a lot of traffic then it might be worth that or more to someone.
I'd post it in the buy/sell section, the serious buyers will contact you. Good luck.
