Science fiction movies are expensive, much more so than more conventional fare. Why? Well, it’s not like you can go rent yourself a space ship or run off to wallmart and buy an NERVA engine, or pick up some aliens from the zoo. If you want this kind of crap, you can rent some of it - there are prop houses in Hollywood that have ‘high tech’ props, but most of these are pretty cheezy, and turn up in so many movies that they’re instantly recognizable as props.