There is no formula.
Some people will say it's 3 months. Other's 6, some 12, and those sitting on gold will not go less than a couple of years worth.
In the end it comes down to what someone is willing to unload it for, and what someone else is willing to pay. The end.
A good domain's nice. Exclusive content, and a few years of it, all the better. Rebilling members, monthly revenue, and traffic are the crown jewels.
In the end it's subjective to numerous variables beyond that, ending in the buyer and seller.
