This is what the blog on touringplans.com has to say about it:
"Decorate your Room – If you are staying at a Disneyland Hotel, the Dream Makers (they’re very handy those Dream Makers) can have your room decorated for you! They need at least 72-hours notice, and these are not cheap options! They start around $200 and work their way up to $400. The least expensive option includes a personalized package from Mickey, a banner on the wall, and balloons in the room. Mickey even leaves a pre-autographed autograph book! The most expensive option includes not just a personalized package from Mickey, but some from his friends too! How excited would your child be to step into the room and find the whole room decorated?"
It sounds like most the of price goes towards the gift package, and I believe for those they have several different themes to choose from (princess, pirate, baby, wedding, chocolate, etc), and each set contains some souvenirs that fit the theme (plushes, autograph books and pens, ears, beach towels, candy, etc).
Hopefully someone comes by with some actual experience, though!