For once, Mystery Science Theater 3000 fans aren't laughing. When the SCI FI Channel announced that it would cancel MST3K after this, the 10th season, MSTies got serious: Efforts to save the show include more than a dozen ''Save MST3K'' websites, a letter-writing push, and a pledge drive for ''Save MST3K'' print ads. ''Our goal is to get SCI FI to reconsider,'' says MST3K Info Club liaison Brian Henry. But according to senior programming VP Bonnie Hammer, the problem isn't just low ratings but rising costs. Film distributors have begun charging as much as $75,000 for movie rights. Which is why fans are also canvassing other networks to pick up the show an approach that worked in '96 when Comedy Central dropped the program. So far, no one's biting, and MST3K host Mike J. Nelson says he's beginning a job search: ''I'm hoping to find a rich guy to just keep me in his living room and heckle live.''