SO MIKHAIL GORBACHEV, perhaps noting Ronald Reagan's travail, has raised the price of a summit. But how did Americans ever talk themselves into believing that a summit with the Soviet leader was something that needed to be paid for, anyway? It's an absurd idea. Not for the first time an administration has incautiously let itself view a summit as an achievement in its own terms and as a political tonic for a struggling president. In this way the Reagan administration baited a little trap for itself, and Mr. Gorbachev has predictably sprung it.

Let no one think, however, that the Soviet leader has done something indescribably brutish that requires a "tough" American response. Mr. Gorbachev is under no obligation to do Mr. Reagan a political favor. The Kremlin leader is presumably pursuing the Soviet interest. For several years nothing has been clearer in Soviet policy than the determination to bring about limits on the American Strategic Defense Initiative. Some Americans may have let their attention stray from this Soviet preoccupation. Mr. Gorbachev has, we trust, put an end to this sort of wishful thinking. There is a school of American opinion that prizes agreement with the Soviet Union -- not to speak of a summit -- so highly that it would have Mr. Reagan rush out now to deliver the requisite concessions to Moscow. But this is no way to run a policy. First of all, for a summit -- a mere meeting -- Mr. Reagan should pay nothing. Then, he must make the decision that he has resisted making ever since SDI opened up for him the shining vista of a nuclear-free world. He must decide whether the vista is close enough to being realized to justify his rejection of the great gains -- great by all past arms-control measures -- that are now apparently available from the Soviets.

One part of Mr. Reagan's mind and administration favors taking the deal Mr. Gorbachev offers, a deal providing room for all the SDI progress that many experts believe to be feasible. We think this is the right course. The other part of Mr. Reagan's mind and administration urges him to reject the Gorbachev deal and to pay any price necessary to make SDI a reality. Mr. Gorbachev was never going to let Ronald Reagan avoid this choice, and now it is before the president in starker terms than ever.