As Greece girds for elections next month that could lead to its exit from the euro zone, economists are acknowledging an unsettling reality: No one knows what the bill will be.

A wide range of potential price tags has been reported, anywhere from €150 billion to €1 trillion euros ($1.27 trillion). But none of these are comprehensive, nor are they meant to be—they don't, for instance, weigh the cost of an exit against the cost of avoiding one. By comparison, the 2008 Troubled Asset Relief Program, known as TARP, was a $700...