I recently used the College Board’s “estimated family contribution” calculator to see how much we’ll be expected to pay when our (currently pre-teen) daughter heads off to college.

The answer? Roughly half our annual incomes. Each year.

No colleges actually charge the amount we’d theoretically be expected to pay. So our out-of-pocket costs would be somewhat less. But the exercise drives home how important it is to run these numbers, early and often, if you want a college education for your kids that doesn’t bankrupt you, and them.

Because I know how the formulas work, I was able to tweak some numbers to lower our EFC. Moving more money into retirement accounts and using savings to pay down the mortgage helped a lot with the federal formula, and helped some with the institutional formula (which, unlike the federal, counts home equity). We still wouldn’t get any need-based help from most colleges but could get some breaks if our daughter gets into one of the most-expensive elite schools. (The total cost of the average public college is $20,000 to $25,000; $40,000 for privates and $60,000 for elites.)

If we didn’t have a fat college savings account, we likely would steer our daughter toward public schools or privates willing to offer merit scholarships to reduce the total cost. It’s much better to start a college search knowing what you can afford than to have to tell your kid, dream school acceptance letter in her hand, that you can’t send her there. Or worse, that you will–and then never be able to retire.

For more about how financial aid formulas work, read my Reuters column this week: “A guide to figuring out the real cost of college.”