Thirty-seven years after two atomic bombs exploded over Japan, people in this country and abroad are thinking hard again about nuclear weapons. Often overlooked in debates between governments and popular movements worried about the bomb is the fact that atomic weapons are relatively cheap.

It is much less expensive, for example, to install a few nuclear-tipped missiles and call it a force that will keep the Soviets at bay than it is to raise powerful armies, navies and air forces that might also keep the Soviets at bay with conventional weapons.

Understanding the role that money plays in how societies decide to defend themselves is crucial for coming to grips with the nuclear peril that many people seem to be feeling more acutely these days.

Specifically, it is important to understand that role in two immediate issues.

One involves the debate over the wisdom of NATO's longstanding policy of threatening the first use of atomic weapons in the face of a potentially overwhelming Soviet conventional attack in Western Europe.

The other involves the forthcoming decision in Washington to deploy new MX intercontinental-range missiles on American soil.

Since the end of World War II, our European allies and Japan have chosen to live rather safely under the protective umbrella of American nuclear-armed missiles and bombers rather than raise big armies of their own to defend their territory.

The concept of nuclear deterrence -- in simple terms, the threat of an atomic counterattack with unimaginable consequences -- has undoubtedly helped keep the two superpowers from a direct clash. But for Europeans, it also has meant defense on the cheap. It was easier to rely on the U.S. nuclear threat -- and a lot less expensive -- than to try to match Soviet conventional power along the East-West border.

Any idea of trying to match the massive Soviet conventional forces was junked by NATO in the early 1950s, when the United States had a comfortable strategic nuclear superiority. Although that clear superiority has been gone for many years, the mentality of not matching the Soviets on the ground has not changed much.

Several European allies -- the West Germans, the French and the British -- have sizable and very good conventional forces. But NATO countries clearly could do much more in their own defense and, by doing so, considerably reduce the likelihood that NATO would have to resort to early and virtually automatic use of U.S. atomic weapons.

U.S. Army Gen. Bernard W. Rogers, the NATO commander, recently said that if NATO governments increased their defense spending beyond current goals by 4 percent a year in real terms (after inflation is taken into account) for the next five years, the alliance would have a chance of defending its territory against conventional attack with conventional, rather than nuclear, forces.

Rogers estimates that this would cost $23 a year for every man, woman and child in the 16-nation NATO alliance. The chances of that happening, however, are very small.

There are several reasons for this, not the least of which is that most Europeans are not nearly as worried about a Soviet military threat as is the Reagan administration. But money is another important factor. Nuclear deterrence is cheaper. So if you don't believe the Russians are coming anyway, it doesn't matter how you deter.

However, if Europeans are really concerned that nuclear war could come and that their homelands would be the first battlefield, then the question of buying their way out of an automatic first-use strategy would seem worthy of serious consideration.

In this country, President Reagan is about to decide on a go-ahead for the MX missile. He is expected to recommend that it be installed in new underground silos in the western United States in a controversial basing scheme called "dense pack."

There are a number of specialists outside the administration who believe that the United States ought to begin getting away from land-based missiles, which are inviting targets, and put more emphasis on basically invulnerable missile-carrying submarines and high-performance bombers.

A respected group of outside defense experts recently recommended this course after a two-year study on "Rethinking the U.S. Strategic Posture" sponsored by the Aspen Institute.

There are many aspects to the debate over land-based missiles and whether they should still play the central role, as they have for 20 years, in America's nuclear triad of bombers and land- and submarine-based missiles. Land-based missiles also have many supporters among military experts.

But one factor in that debate that doesn't get much attention is money. It is true that deployment of 100 MX missiles will be expensive -- at least $25 billion over the next five years or so. But once they are installed in their silos, land-based missiles are very cheap to keep operating for 20 years because basically they just sit there.

Undersecretary of Defense Fred C. Ikle recently reminded some journalists of this point, among other things, when asked why land-based missiles should be retained.

In contrast to keeping missile-firing submarines with 133-man crews at sea and fuel-guzzling bombers in the air, land-based missiles are an atomic bargain that is very tempting for governments, regardless of the strategy questions raised about whether they are vulnerable to attack and even invite attack. There is no doubt that the theoretical vulnerability of both U.S. and Soviet land-based missiles to attack is the central fact that drives the arms race.

So in this country and in Western Europe, where many are worried about where the world is heading on the question of nuclear arms, there may be ways to buy some relief if people are willing to pay the price and if governments are willing to consider new strategies.