Another model, Litter Maid (~$120), uses custom plastic trays. It's cheaper, but it doesn't work as well as the Litter Robot. After a few months you'll find yourself tending to it every other day. The cost of the plastic trays added up over the course of a year, but it's a non-DRM receptacle, so you can hack a cheaper 'solution' at home with a small amount of craftiness. If you do go with Litter Maid, go for the cheaper one- it actually works better than the 'Elite' model.
But really, go for the Litter Robot. I've had mine for two or three years and I love it.