'Why on Earth would you buy down here when you can rent?' asked a friend of mine in Miami Beach not long ago. 'Buying is so over.'

He promptly moved to Manhattan for work reasons-and bought a $1 million loft on the Upper West Side.

Note the typicalbehavior. People want to buy when prices are up, and turn more wary when they've collapsed. Logically it makes no sense. Research out Thursday adds some hard numbers.

Real estate website Trulia.com has looked at major real estate markets across the country and asked: Is it cheaper to buy, or to rent?

By Trulia's math my friend was moving in exactly the wrong direction.

Rent in Manhattan: Home prices there are way too high, says Trulia. (Ditto San Francisco.)

Buy in Miami. And Phoenix. And Las Vegas. And most of the other places that have been flattened by the crash. Homes there are cheap compared to rents.

The cross-over point is about 15 times annual rent, the company believes. In other words, as a rough rule of thumb, homes are probably fairly valued in a city when they cost about 15 times a year's rent. So, for example, if you're paying $10,000 a year to rent a place, think twice about buying a home that costs more than $150,000. Dean Baker, economist at the Washington, D.C. think-tank The Center for Economic and Policy Research, came to a similar conclusion in research on the subject in recent years. Fifteen times is the historic average, he said.

So what's the multiple in New York right now?

About 32 times, says Trulia. The average two-bedroom condo or townhouse in New York city costs about 32 times as much to buy as it does to rent. Other major markets over 20 times include Seattle (24 times), San Francisco (22 times) and Portland, Ore. (22 times).

On the other hand Miami list prices are now about eight times annual rents, says Trulia. Phoenix is about 10 times and Las Vegas about 11.

Trulia's data need to be taken with some caveats.

Trulia looked at list prices rather than actual transaction prices, so its figures for prices may be too high.

Furthermore drawing the cut-off point at 15 times rents may be on the low side.

Mr. Baker, in conversation yesterday, said that figure assumes that you're only going to stay in your home for the typical seven years. If you stay a lot longer, he says, the transaction costs of buying and selling become less and less important. That makes owning more attractive.

Nonetheless the Trulia analysis seems directionally correct. Work done by the C.E.P.R. last year came to similar conclusions: Namely that markets like New York and the California coast remained expensive compared to rents, while the hardest hit markets now look cheap.

And Trulia's research emphasizes two points that are absolutely spot on.

First, homeowners need to look first and hardest at present cashflow. The cult of homeownership made no sense. If renting is much cheaper than buying, think seriously about it.

Second: The markets that have fallen the furthest now look like good places to buy, while those that seem to be 'safest' aren't. As the saying goes: There is no such thing as a 'safe' investment, merely one whose risks are not yet apparent. It's a principle that a lot of people forget time and again.