Costs, simply put, are the sacrifices people make to do something. According to economists, the sacrifices that matter when making a rational decision to do something are those that otherwise would not have been made (the marginal costs). Sometimes these are measurable costs (like a ticket to a movie). Sometimes these costs aren't measurable (like the value you place on watching a television show that you forgo watching to go see a movie) to a third party observer. Not only that, but third parties can't verify such sacrifices. How do I know if you aren't BS'ing me?
But to an economist, it doesn't matter if a sacrifice is measurable by a third party. What matters is how the decision maker internally feels about these sacrifices - the decision-maker's internal value of the thing forgone.
Accountants, on the other hand, prepare documents that describe the financial health of firms for the benefit of owners and for tax reasons. Costs also represent some sort of sacrifice when viewed by accountants, but accounting costs are costs that must be measurable and they must be verifiable. The documents that accountants create must satisfy the generally-accepted accounting principles and tax law.
For example, suppose Jared owns a business and has a son, Alex. Suppose that on one day, Jared chooses to work rather than to spend time with his son. Suppose that Jared internally values that time with his son at $25. To the economist, that sacrifice is an important part of the overall decision to work. But if Jared tried to deduct that $25 as a business expense, he'd be in trouble with the IRS faster than you could say "snail snot." Do you have a receipt for that, Jared?
Because accountants and economists treat some sacrifices differently, they can have differing opinions on the overall health of a business. Here is a link to an article from 2005 on Major League Baseball team profitability statement as compiled by Forbes magazine, a statement that baseball commissioner Bud Selig took issue with. It includes a famous quote by Paul Beeston, formerly an executive with the Toronto Blue Jays, that underscores the difference between how economists think of costs and the costs that accountants typically put in their documents.
Ozanian quoted former MLB president and chief operating officer Paul Beeston when he was an executive with the Toronto Blue Jays: "I can turn a $4 million profit into a $2 million loss and I can get every national accounting firm to agree with me."
The tax laws and the generally-accpeted principles exist for a reason, but they can cloud the real issues behind why people do things.