American tourists grumble about prices in Italy even as they pitch coins into the Trevi Fountain. The symbolic gesture is supposed to guarantee a return to Rome, but that is something fewer Americans can afford to do.

"We've been traveling for a month and this is the most expensive place we've been," said Caroline Faris of Wilson, N.C. "Paris and Nice were cheaper, and that's surprising."Italian tour operators aren't surprised. They say Italy is pricing itself out of the market with exorbitant rates and inferior service. The government predicts that compared to last year, U.S. tourism will drop by more than 6 percent, or 112,000 people.

Tourism from other countries is on the rise, and students and backpackers from the United States still jam the streets of Florence and the ferries in the Venice lagoon. The drop-off is among the big spenders.

"We are one of the most expensive countries in the world for hotels and restaurants. And this comes with a very low level of service quality," said Isabella di Montezemolo, a spokeswoman for American Express Travel Services here.

"Our country is very rich in natural and historical beauty, but people are getting a bit tired of the services," she said. "They feel they are being taken for a ride."

"Our tourist services are 30 years old. They are too old, too small and not competitive anymore," said Enzo Poli, the president of a federation of 2,700 Italian travel agents and tour operators.

Giovanni Colombo, the president of the Federation of Italian Hotels, said that compared to last year, the number of foreign tourists in Italian hotels has increased by 5 percent. di Montezmolo cited a 47 percent increase this year in Japanese tourism due to a favorable exchange rate.

But industry officials are concerned about the drop in the number of Americans, who traditionally are among the biggest spenders.

"This year Americans won't be the big spenders and they will shorten their vacations in Italy," said Franco Cesaretti of the Aviatur travel agency in Rome.

Franco Paloscia, spokesman for the Italian National Tourist Office, said his government spent about $9.2 million over the past two years to promote Italy's image.

His office predicts Americans will account for 6 percent of foreign tourism this year and at least 10 percent of the revenue. In 1985, Americans made up about 7 percent of the tourists and 17 percent of the revenue.