eelixduppy - 6:02 pm on Apr 3, 2012 (gmt 0)
While I agree there is no definitive answer and a real estimate would require a professional, you can get a decent understanding by looking at
1) your current and historical financials (looking at something like operation, financing, and investment totals)
2) your competitors/comparable companies to gauge relative worth/growth/etc
3) your industry/niche as a whole which may be in a specific business "cycle" and as such may affect your current financials
Addressing these points will also help you justify any of your asking prices.