In a sense, the unprecedented gathering here of leaders from 22 rich, poor, and middle-income nations amounts to a very expensive seminar. No great harvest of new proposals for development is expected to result. The gap between the world's rich and poor nations will not be suddenly or easily closed.
But the seeds of ideas may be planted here. And President Reagan may prove the ''seminar's'' main student.
He himself has indicated a willingness to listen and learn; and the leaders of several so-called developing nations seem to regard this as an opportunity to educate the leader of one of the globe's most affluent nations.
Cancun means ''pot of gold at the end of the rainbow'' in the language of the Mayans. But President Reagan has made it clear in advance that the United States is refusing proposals for huge transfers of wealth from the rich to the poor.
''The problems of hunger and poverty are severe and deeply rooted,'' said Reagan in a statement upon his departure on Oct. 21 for Cancun. ''They cannot be solved overnight, nor can massive transfers of wealth somehow miraculously produce new well-being.''
The President repeated his standard message: that prosperity results from economic freedom and individual incentive - a theme some third-world leaders believe cannot adequately answer their problems.
At the same time, Reagan backed away from a statement he made about a week ago. He had said that he expected to encounter a hostile environment in Cancun. Asked about this on his departure from Washington, he said that the word ''hostile'' was perhaps too harsh.
That change in attitude and apparent willingness to listen reflects the US leader's determination to avoid getting trapped in acrid confrontations. In addition, the two days of meetings between the 22 leaders - representing some two-thirds of the world's more than 4 billion people - have specifically been designed to avoid confrontation and to encourage relaxed give-and-take.
There are to be no lengthy formal statements, resolutions, or communiques. Exposure to the press is being kept to a minimum. If any leader wants to make Cancun a stage for dramatic proclamations, he or she will have to wait until after the meetings are over.
The participants are holding their more than 12 hours of talks Oct. 22-23 at the secluded Cancun Sheraton Hotel. Shaped like a Mayan pyramid, it overlooks a 400-year-old Mayan temple and 10-mile stretch of white powdered limestone beach. Mexican security men have closed off the hotel as well as the two bridges that connect Cancun Island with the Mexican Caribbean's Yucatan Peninsula.
Reagan administration officials have indicated that they would like to see ''follow-up'' to Cancun in the form of meetings of specialists within existing international economic and financial organizations. They do not favor another Cancun-style summit.
They are opposed to ''global negotiations'' under the United Nations umbrella which have involved more than 100 nations and in the Reagan view produced little more than strident rhetoric. But administration officials have also softened their opposition by saying that the US is ''prepared to engage in preparations leading toward global negotiations.''
While all this is likely to be of little consolation to the many developing nations that favor global negotiations, it does reflect a more positive attitude toward economic development talks than that which characterized the Reagan administration earlier on. For one thing, in studying the problem administration officials have come to recognize just how dependent on the developing nations the United States is for its own prosperity.
In a breakfast meeting with reporters shortly before President Reagan's departure for Cancun, Robert B. Hormats, Assistant Secretary of State for Economic and Business Affairs, indicated that there is more ''give'' in the Reagan position than has perhaps been publicly acknowledged.
He said that, taking into account a variety of new US ideas mainly in the area of trade rather than aid, the US had a ''positive program.''