(FORTUNE Magazine) – The 38th annual Cannes Film Festival opened with a distinctive American flavor: four of the 19 films shown officially were made in the U.S., compared with two last year. American filmmakers flocked to the festival even though the strong dollar has flattened their income from overseas sales. Foreign movie-house rentals accounted for only 18% of total film rentals last year, compared with 29% in 1978, according to a study by the Wall Street firm of Wertheim & Co. While all rentals rose 8.6% last year to about $4.5 billion, the cost of filmmaking is skyrocketing. The average film budget last year was $14.4 million, up 17% from 1983. Spending more on a film does not necessarily mean earning more. According to the Wertheim study, 27 of the 34 films made in 1984 that cost more than $14 million lost money or barely broke even. This year's most expensive picture will be Santa Claus, a new version of the old story -- starring Dudley Moore as an elf -- produced for around $50 million by the same group that created the Superman movies. Santa Claus, promoted at Cannes with a film clip, will be released in November. The company releasing Santa Claus, Tri-Star Pictures (one-third owned by Time Inc., publisher of FORTUNE), is hoping that the film will snowball at the box office. A failure would put a freeze on big film budgets that have already been cooling. Says Alexander Salkind, executive producer of Santa Claus, ''The studios generally don't like to make big-budget pictures anymore.'' If his film flops, it could be tough sledding for more than Santa this year.