Basically there isn't a single way to define cost, which is quite inconvenient for us. What you're going to use the cost to figure out should tell you how to calculate it however.
Let's start with some recapping from the last section.
Let's say that a company's average debtor payback time in 2008 was 62 days, and in 2009 was 31 days. A possible reason for this is that the company has improved their credit control procedures, meaning they now collect their debts more quickly.
Some things that are identical have different values.
An empty tin of dog food is worth about $0.10. An empty tin of Paris Hilton's dog food went on ebay for $305.
I think the message here is that most people are crazy, but there is probably some sort of deeper meaning too. If you manage to work out what it is, please let me know.