One of the best features is the night visions (even on cheaper models) which makes it easy to see what is going on in a dark room. (childproofingtips.com)
You can, indeed, get cheaper models, though, with very good sound quality, to boot. (tomsguide.com)
However, whether measures like these really help remains to be seen; an even cheaper model — at around say $ 300 — could have helped build demand even more. (goodereader.com)