(FORTUNE Magazine) – If you're the sort who is tantalized by brain teasers, try this one: How much money did eBay make over the past five years? That may sound like an odd question. But trust us, it isn't. "For the life of me, I wouldn't be able to tell," says Albert Meyer, an accounting sleuth who runs 2nd Opinion Research.

The reason (of course) is stock options. As anyone knows who's followed the vicious post-bubble debate over this issue, measuring the cost of these controversial goodies is devilishly complex. Ah, you say, but surely when the Financial Accounting Standards Board finally requires companies to expense options, as it proposed to do this spring, then some kind of clarity will emerge. Don't count on it. As the case of eBay demonstrates, even after expensing kicks in, the debate about what options really "cost" and how to measure them is likely to rage on. Maybe even fiercer than ever.

Let's pick on eBay because, while it is far from alone, the sheer scale of the numbers and its sky-high stock price make it an illuminating example of what we'll call the Profitability Uncertainty Principle. Start by adding up eBay's reported net income from 1999 through 2003: $840 million. Under current rules this doesn't include stock options--a major cost. For those you must turn to the footnotes in eBay's annual filings. There you learn that after deducting $827 million worth of options, eBay has earned just $13 million during the past five years. Sure, that number is hard to find, but not for an expert like Albert Meyer. So why would he say he's still confused about what eBay really earned?

First, because even that calculation--done using one of the standard valuation mechanisms like the Black-Scholes model --purports to measure only what the value of the options was when they were granted. There's another way to look at it. Meyer calculates that employees pocketed $1.6 billion in option-based compensation over this five-year period. So did the options "cost" $827 million or $1.6 billion? (Consider that $1.6 billion is the cumulative figure that eBay has claimed as an expense on its tax returns.)

Second, even if you can figure out what net income actually is, stock options can also meddle with cash flow. Steve Milunovich of Merrill Lynch thinks that operating cash flow should exclude what companies spend to buy back stock to offset option-related dilution. This gets tricky in eBay's case, because the company, unlike many others, hasn't bought back any stock. In other words, even if you're a stockholder who never sold a share, you own less of eBay now than you did in 1999. Meyer calculates that had eBay bought back shares each year to enable stockholders to maintain their ownership stake rather than seeing it slowly whittled away, the cost would have been $1.2 billion--which is a big number relative to Bay's cumulative operating cash flow of $1.8 billion.

eBay has its own point of view, which is that--surprise!--most measurements of the cost of options are flawed. But chief accounting officer Mark Rubash does believe that option expense should be reflected (to the extent that it can be reasonably measured). He says dilution is what matters to most investors, and so companies should consider providing a "dilution statement," laying out which employees got what and how much it cost existing shareholders, along with the balance sheet, income statement, and cash flow statements. (Rubash also points out that as eBay matures, the company is increasingly paying people in cash.)

Debate aside, under FASB's proposal to expense options, eBay would have had to report its five-year net income as $13 million. Jack Ciesielski, who writes the Analyst's Accounting Observer, argues that that's the right number and that it doesn't make sense, after the fact, to adjust the cost to reflect what the company spends to buy back stock or what employees actually pocket.

And that's FASB's position--well, pretty much. Mike Tovey, practice fellow at FASB, does note that there's a project underway to reconsider what is a liability and what is an equity instrument. "It's not inconceivable," he says, should options be reclassified as the former, that FASB may eventually require companies to reflect changes in the values of their options on the income statement.

So does the fact that expensing will likely intensify these tortured debates rather than end them mean we shouldn't bother? No, because while the right number might be controversial, it's perfectly clear that there's one absurdly wrong number: zero.

Perhaps we should take our inspiration from modern physics. Doesn't the mere act of measuring something change it? Maybe when investors are finally forced to measure the cost of options, the game will change. Maybe companies will switch back to cash.

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