A simple way to do the comparison is to look at something called the rent ratio: the purchase price of a house divided by the annual cost of renting a similar one. The number 20 provides a useful rule of thumb. When you do the math, you discover that a ratio above 20 means you should at least consider renting, especially if you may move again in the next five years or so. When the ratio is well below 20, the case for buying becomes a lot stronger.

In many large metropolitan areas, including New York, Los Angeles, Chicago, Houston, Dallas, Atlanta and South Florida, the average ratio is now 16 or lower. It was more than 25 in several of these places at the peak of the bubble, about five years ago. With a ratio as low as 16 and interest rates as low as they are, the costs of owning can be less than the costs of renting — and buyers will end up worse off only if prices fall considerably more.

A two-bedroom Spanish-style condominium in Beverly Hills, Calif., for example, recently went on the market for $1.075 million, notes Don Heller of Prudential California Realty. Including taxes, condo fees and the tax deduction for mortgage interest, a typical buyer making a 20 percent down payment would face an effective monthly payment of about $6,000. Compare that with the monthly rent on a similar two-bedroom condo nearby — $7,600.

The math works out similarly in less costly areas, too, be it once booming cities like Phoenix and Orlando, Fla.; Midwestern cities like Minneapolis and Cleveland; or the outer-ring suburbs of most big cities. Much of New York’s outer boroughs appear to fall into this category.

The problem for potential buyers is that many real estate agents argue for buying even in places where the numbers don’t add up. In the Bay Area, the rent ratio remains around 30. In Seattle, it’s about 28. In parts of Manhattan, it appears to be about 25, according to current listings.

“In most markets, you’re better off buying,” Thomas Lys, an accounting professor at Northwestern University, says. “But once the ratio gets to 25 or 30, I’d say, ‘You know what? There may be a bubble.’”

The rent ratio has long been higher in New York and San Francisco than most places, perhaps because of zoning rules or because the cities are home to large numbers of affluent households willing to pay extra to own. So it’s possible that prices will not fall. But they are already high enough that the monthly costs of owning often exceed the cost of renting — even without taking into account the down payment or other one-time costs.