wheel - 4:18 pm on Oct 19, 2011 (gmt 0)
Probably something to do with income/revenue/profit. Some multiple like 6 months to 2 years.
But depending on the market, the reality will vary wildly and will depend much more on factors like how much of the market you own, and the willingness of potential buyers to pay. So use a formula, but if you're selling, don't trust it as being a true value.