To be honest if your water usage from a bidet is high enough to be significant as part of your overall bill or usage, you are doing a pretty damn good job. Also, might I recommend taking a shower sometime. :P
Learned another benefit today. If you're having digestive problems because, say, you ate tainted meat, cleaning up is dramatically less painful with a bidet the four or fifth time you visit the restroom.
TMI and all, but I want to make the strongest case I can here. This is really some of the best $30 I've ever spent. I would say if you count intangibles we've already broken even.
To the person who was asking about getting poop on the bidet, you'd clean it just the same way you clean the rest of the toilet. While not in use, this one is retracted, though, so you'd need to be pooping at a fairly creative angle to hit it.