In the Thai border town of Aranyaprathet, an American medical coordinator rents a small wooden house for $350 a month and tell friends he has a bargain. Before doctors and relief workers descended on the town to work in nearby refugee camps, the landlord would probably have got $50.

On a road traversing a refugee settlement, a teen-aged Cambodian approaches a journalist and asks him to cash a $105 money order drawn on a small-town American bank. Relatives in the United States mailed it to him, he explains.

In Bangkok, foreign relief agencies buy more than $50 million of rice and so much cooking oil that shortages occur. The agencies pay wages to hundreds of Thais employed as social workers, dirivers, secretaries and press officers. o

Every day, transsactions such as these give evidence that war in Cambodia and the Indochinese exodus have channeled to Thailand a torrent of dollars and gold.

The country at the same time faces a host of tangible and invisible costs -- increased military spending in the face of Vietnamese troops on its border, for instance, and political murmurings among Thais about the wisdom of allowing almost 300,000 Indochinese into the country.

Viewed in purely commercial terms, manly specialists believe Thailand is coming off quite well. Foreign purchases for refugees and the Cambodian relief effort, private remittances to refugees and illicit sales to Cambodia could inject about $400 million into the economy in 1980.

"That would be a conservative estimate," a prominent Thai economist said. The sum is approximately 7 percent of projected export earnings for 1980. The added income will cushion Thailand from the world recession and improve its ailing balance of payments figures.

There are negative effects as well, however: new opportunities for corruption in Thai officialdom, two to three percentage points added to the 20 percent-plus national rate of inflation and even higher inflation in some border communities such as Aranyaprathet.

The joint Cambodian relief effort launched last fall by U.N. Children's Fund and the International Red Cross will have cost about $500 million by the end of 1980. About $200 million ofthat will have been spent directly in Thailand, according to agency sources.

This includes about $100 million for 150,000 newly arrived Cambodians in camps inside Thailand; about $50 million for rice bound for Cambodia or camps on the border, and about $10 million for 200 trucks and 75 tractors and barges for use inside Camabodia.

During 1980, the U.N. Office of the High Commissioner for Refugees will spend about $20 million feeding another 140,00 Indochinese refugees in older Thai camps. It will also disburse about $15 million to programs aiding 200,00 Thai citizens said to have been displaced by the refugee influx and fighting.

In addition to U.N. agencies and the Red Cross, more than 50 private voluntary agencies operate in Thailand,many of them self-financed. Foreign embassies have brought in extra staff for relief and resettlement work -- about 85 Americans are assigned to this duty in the U.S. Embassy.

In many cases, Thai officials direct spending and services toward Thai citizens. Medical teams working in border camps are encouraged to set aside certain days to receive Thai villagers. In construction of new camps, use of Thai labor is encouraged, although refugees would work for far less.

In designing three new U.N. financed "holding centers" for Cambodians, Thai officials insisted on more expensive and durable housing than the U.N. had planned. "they wanted houses that would outlive the refugee problem," one refugee worker said. "the three camps are all well-positioned for possible use by the military later on."

Relief workers sometimes complain that funds are siphoned off by corrupt Thais using some of the same ruses that plagued the former government in South Vietnam.

Camp commanders pad the rolls with nonexistent "phantom refugees," it is said, then pocket money received for their upkeep. Rice shipped to the camps is sold and replaced with substandard varieties. Guards charge refugees for permission to leave the camp to work, foreigners allege.

In theory, the camp resident needs no money of his own. (nonetheless, thousands of refugees with relatives in America or Europe regularly receive checks from them. Cashed at local banks or sold at large discounts to money-lenders, the funds enter the local economy.

At one camp of 13,000 refugees from hill tribes, foreign relief workers provided a check-cashing service to the refugees and were soon handling more than $5,000 a week. At the larger camp at Norg Khai, in the Mekong River, the business is "absolutely colossall," one foreigner said.

It is not possible to compute how much money enters Thailand this way, nor how much Thai merchants bring in trading with Cambodians who remain in their own county. But since shortly after Vietnamese forces overthrew the Khmer Rouge government of Pol Pot, gold, silver and foreign currency long buried in flowerpots and back yards has been flodding into Thailand illicitly.

In return, Thai black marketeers provide sarongs, shoes, bicycles, scented soap, cigarettes, tape cassettes and any other consumer goods the Cambodians can pay for. Journalists traveling inside Cambodia gain the impression that the entire population dresses in Thai clothes.

"The trade has taken up the slack," the Thai economist said. "The world recession has not affected certain sectors [of the Thai economy] as hard as it might have. Textiles, for instance -- border traffic has kept the looms going at an even pace."

It has meant enormous profits for Chinese entrepreneurs who now make their headquarters in hotel rooms and shops in border towns like Aranyprathet. tGoods can be seen stacked in bundles on sidewalks all over town.

But coupled with the arrival of free spending foreign aid workers, the boom has brought triple-digit inflation to some sectors of the local ecomomy, outstripping the incomes of ordinary families and forcing many to try their hand at blackmarketing themselves. Carrying sacks of merchandise over their shoulders, they risk arrest and sometimes death at the lawless border camps where trading takes place.

A respected Thai busines magazine has estimated that in 1979 about 300,000 ounces of gold found its way to Bangkok from Cambodia, at current prices worth about $200 million. In 1980, however, gold flows have slowed considerably as Cambodian stocks run out. Silver is now more common.

None of these financial benefits, however, means that the Hai government wants refugees to remain. On the contrary, the prevailing view in government offices is that they present a long-term threat to prosperity and security.

Officials often point out that in the 1940's, Thailand allowed about 50,000 Vietnamese to cross into its northeastern provinces to escape fighting between the French and Viet Minh.

Thirty years later, most of them are still here. They dominate trading in district towns and are believed by the Thais to assist local communist insurgents and relay intelligence on Thai military strength to Hanoi.

Thai officials question foreign agencies' claim that aid covers all but a fraction of the cost of supporting the new Indochinese refugees, and say there remains a heavy financial burden on Thailand.

A spokeswoman for the Ministry of Interior, for instance, said it had assigned 200 persons to work full time with refugees at a cost of $1 million per year. The Thai Army and provincial governments have an unknown number of personnel in this field as well.

Thai expenditures, or losses for wage, camp sites, vehicles and other overheads came to $6 million in 1978, according to official publications.

Vietnam's invasion of "Cambodia, which Bangkok blames for the refugee overflow from that country, has forced Thailand to augment its military budget, it is argued. Defense spending is reportedly scheduled to rise by about $300 million to $1.4 billion in the Thai fiscal year 1981.

There are hidden social and security costs too, the government claims: the danger of fifth columnists entering as refugees and the resentment of Thai villagers who see people getting free housing, food and medical care and the chance to emigrate to the United States.

Officials play down any financial benefit that Thailand may reap from Indochina. It is feared that the United States and other countries of resettlement would be less quick to fly refugees out if it was believed that Thailand was profiting from their presence.

Thailand also worries that in coming years it will face more of the burden of supporting the refugees itself.

"How long will Cambodia keep the world's attention?" one government official asked.