HOW MUCH FOR “LEGEND”?: Even though Jim Cameron and Fox will survive the $200 million “Titanic,” studios finally seem serious about curtailing runaway costs — even on the seemingly irresistible pairing of Ridley Scott and Arnold Schwarzenegger in a science-fiction setting.Warner Bros. executives are choking on the budget of “I Am Legend,” which is hovering at around $108 million. The film is an adaptation of the Richard Matheson cult novel in which a biological holocaust leaves Schwarzenegger the last human to survive. The others have mutated into fearsome, blood-sucking, night-dwelling mutants. The story was previously filmed as “The Last Man on Earth” and “The Omega Man.”But days after the Phil Alden Robinson-directed Harrison Ford starrer “The Age of Aquarius” fell into jeopardy and lost its star because of delays on a budget that hit $94 million, it is becoming clear that “Legend” might not see the light of day unless the budget can somehow be brought down to $90 million. This is despite the fact that Scott directed two revered sci-fi films — “Blade Runner” and “Alien” — and Schwarzenegger has toplined such sci-fi blockbusters as the two “Terminator” films and “Total Recall.” Despite attempts to cut costs, the film is undenia-bly expensive — just re-creating a post-apocalyptic L.A., destroyed and devoid of people, is a costly logistical nightmare. A Scott-Schwarzenegger sci-fi package seems about as safe a big-budget bet as a studio could make, but sources said media and shareholder scrutiny of films whose costs exceed $100 million has left execs all over town in cold sweats. LEIGHTON GOES SITCOM ROUTE: After five seasons as the vixenish Sydney on “Melrose Place,” Laura Leighton’s ready to try comedy. She has signed a deal with producers Bob Greenblatt and David Janollari to develop a sitcom under their deal with Twentieth TV. Leighton is already warming to the laugh track. She’s co-starring in the indie film “Sky is Falling” with Dedee Pfeiffer, Teri Garr and Howard Hesse-man, a film written and directed by first-timer Florrie Laurence. Leighton’s role provides the comic relief. While Greenblatt and Janollari still have to come up with a concept for Leighton, she demonstrated remarkable range on “Melrose”: She poisoned her sister; stole not only sis’s husband but also her busi-ness; was institutionalized; blackmailed half the Melrose tenants; romanced and dumped a Trump-like zillionaire; and engaged in numerous catfights that led, in separate incidents, to her being hurled in a wedding dress over the roof of a car and into a swimming pool. While she waits, Leighton has also become spokeswoman for the National Panic Disorder Art Project campaign, benefiting the millions afflicted by panic attacks. Leighton is repped by UTA. “GRATEFUL” PARTICIPANTS: Showtime has assembled a cast of Ted Danson, Steven Weber, Marg Helgenberger, Jennifer Jason Leigh, Brian Dennehy and Matt Keeslar for “Thanks of a Grateful Nation,” a film that focuses on the effects of Gulf War Syn-drome. The film was written by John Sacret Young (co-creator of “China Beach”) and directed by Rod Holcomb (whose TV credits include “ER” and the “China Beach” pilot). It’s being produced by Tracey Alexander and Andrew Adelson and Young. Because of the continuing standoff over chemical weapons inspections with Iraqi leader Saddam Hussein, the project is timely, though Young and Alexander have been working on it for four years, from the time they optioned a Redbook article by David France. The story was about a number of women whose husbands returned from the Gulf War and sexually transmitted strange maladies such as urinary tract infections and rashes. The suspicion was that soldiers were suffering from exposure to the bombing of chemical plants in Iraq. At the time, said Alexander, the U.S. government acknowledged one case, but since then that number has climbed to 100,000 and is still rising. “This says a lot about the future of warfare,” said Alexander. “It looks great on television, but the lingering after-effects don’t get any coverage.” Alexander said the film is also an indictment of the dealing of chemical weapons to countries like Iraq. “There’s documentation that proves an American company legally sold agents for anthrax and botulism to Iraq before the war, as scary as that sounds.” The film, overseen by Showtime’s Joan Boorstein and prexy Jerry Offsay, is filming in Toronto for a late May airing.