I believe the stroller hire is about 7 euros a day.
You cannot take them out of the park, if you want to park hop, you hand the stroller back in one park and get a receipt to hand over at the other one, so you don't pay twice.
There is a means of hiring the buggy for a longer term and you pay a deposit, it was 75 euros back in 2006 plus the daily rate, or at least that's what we thought the CM said, (her English wasn't very good and our French is awful), but as another guest was at the same time trying to get her deposit returned and having problems explaining her situation, we decided not to bother and hire every day instead.
There aren't any double buggies available, but the older style ones as in Reid's picture above have a buggy board, previous visitors have reported that it is getting more difficult to hire these as they are being replaced with a new 3 wheel version that doesn't have the board.
Yes, they do go missing, only once have we ever needed to hire one and I lost track of the amount of times ours disappeared, in the end my sister resorted to tying her scarf on the handle and it seems that whilst people are happy to pinch your buggy they drawer the line at removing your personal possessions first as that seemed to do the trick and we didn't lose it again afterwards. So my advice would be take something that you're not too worried about losing that might act as a deterrent. CM's do tend to have a tidy up as well so if you come out of a show you may find it's moved. You can get a replacement with your receipt but walking back to the park entrance to get one does wear a bit thin after a while.
At the end of the day people seem to leave them all around the park, CM's obviously do a night time collection.
Hope this answers your questions.
Jill.