Craig - I`m not a financial guru nor do I play one on TV. But I have been in business for many years - for others and for myself. I don`t believe there is any one "magic" formula for ALL products or ALL industries. I believe eBay examples are entirely different than brick & mortar and even compared to service type businesses.
If you`re in business selling product(s) and have a benchmark of 40% GP and have operating expenses at 38%, you`re at a 2% Net. If you`re doing $50 million annually at 2% net, you made a cool $1 million. But if you`re at $50K annually at 2% net, you made $1000. Same factors, different results.
What the market can bear many times dictates price. The bottom line ... the seller has to determine what they can live with based on their own limitations, restrictions, requirements, etc. I`m sure you can find "norms" in most industries, but a "one size fits all" formula ... I seriously doubt it.
Speaking of money ... that`s my $.02 worth 
R@
