House shoppers will have to buy soon to secure an ultra-cheap mortgage and collect an $8,000 tax credit. Both perks are expected to vanish in the coming months. However, some buyers might score better deals by waiting and missing the perks on purpose, precisely because so many others are buying now.

To see why, let s first review the incentives. For about a year, the Federal Reserve has been aggressively buying mortgage securities at generous prices to drive down yields, and by extension, to reduce rates on new mortgages. Those purchases end in March. Most economists, including Boston Fed Chief Eric S. Rosengren, reckon rates on standard 30-year mortgages will rise about three-quarters of a percentage point thereafter.

Congress has been propping up the real estate market, too. In November, lawmakers extended and expanded a refundable tax credit (a rebate, essentially) for house buyers. Buyers are given 10% of the purchase price up to a maximum credit of $8,000 for first-time buyers and $6,500 for repeat buyers. They must sign a purchase agreement by April 30 and take ownership by June 30. Income limits apply. The credit is reduced for individuals who make more than $125,000 a year and couples who make more than $225,000, and it's eliminated for individuals making $145,00 and couples making $245,000.

Economics students know that subsidies increase demand and raise prices. Longstanding perks for house buyers like the mortgage interest write-off and the artificially reduced down payments for first-time buyers were designed primarily to expand homeownership. If they increased prices, it was an accidental byproduct. But the two temporary perks were designed for the express purpose of supporting prices by attracting buyers.

They worked. Goldman Sachs reckons that government support boosted house prices by 5% last year or rather, kept them from falling that much more. It seems reasonable to assume, then, that prices could lose 5% once the programs expire. And because economists believe the temporary incentives mostly convinced people who were already considering buying to do so sooner rather than later, when the programs expire, buyers could disappear and prices could dip quickly. So potential buyers should consider whether they re better off hurrying for the perks or waiting for the lower prices once houses lose their stimulus premium.

If this were 2007, the decision would be easy. In April of that year, I argued here that renting had become a better deal than homeownership. Prices were so bloated that there was little point to considering differences among individual markets or the value of incentives. But since then prices have fallen by more than one-third, adjusted for inflation. In July, I revisited the subject using May data, and concluded that prices were almost but not quite back to normal, and that some markets that had been hit hard by price declines looked cheap. That s more or less still the case.

Let s assume, then, that house prices where you live are fair. Are you better off with an $8,000 tax credit and an interest-rate reduction of three-quarters of a percentage point or with a 5% price reduction? For cash buyers, the math is easy. For houses priced below $160,000, the $8,000 subsidy being offered now is greater than the likely discount later. Of course, most buyers borrow. For them, the math gets trickier. One way to approach it is to pretend the forfeited 5% price decline for early buyers represents points on a mortgage. Mortgage lenders usually let buyers pay percentage points of the purchase price upfront in exchange for lower interest rates over the life of the loan.

Suppose you buy a $300,000 house in February, and by doing so you forfeit a 5% price decline, or $15,000. You get the $8,000 credit, for a net forfeit of $7,000. Now suppose you put down 20% as a down payment and take out a mortgage for the $240,000 difference. The $7,000 you gave up was 2.93 points, when divided by the mortgage amount. In return, because you bought before the end of March, your interest rate is 5.16% (the current average) instead of 5.91% (the three-quarters of a point increase expected after March). Should you pay 2.93 points for a rate reduction of that size on a $240,000 mortgage? Use SmartMoney.com s Points or No Points calculator to find out. You ll have to assume a rate of return the buyer gets on investments under normal circumstances. I used 5%, and got a result of seven years. That s the breakeven point. If you plan to stay in the house for that long, you should pay the points or in this case, you should buy sooner rather than later.

For a $500,000 house using the same methodology, buying early is like paying 4.25 points, which gives a breakeven point of 11 years, 11 months. For a $700,000 house, it s 4.82 points, which breaks even after 14 years, eight months. But things get tricky for pricier houses. The Fed s buying of mortgage securities has mostly suppressed rates for what are called conforming mortgages. On single-family houses, these are loans of $417,000 or less in most areas and, for the moment, $729,750 or less in designated high-cost areas. For nonconforming loans, rates are already higher, so they might not rise much after March. Also, the $8,000 tax credit vanishes for houses that cost more than $800,000.

Obviously, this is a crude analysis. Houses prices might fall less than 5% after the perks disappear, or they might fall more. Price changes will vary across regions and according to starting house values. And we won t know for sure what portion of future price movement is a result of lost subsidies, per se. But the point is that, just as there are incentives to buying quickly, there are offsetting incentives to waiting. For many buyers, especially those buying more expensive houses and those who might move in fewer than 10 years, waiting for the perks to expire will likely be a better deal. No matter what, don t buy a house you don t love at a price you don t love just because the government is waving a check.