Yes they are expensive but they work great and your kids will love them.
Our 3 & 4 yr olds love helping us make the mix and then can go right to the fridge and take one when they want. We make it with fruit and milk with no added sugar so they make for a great snack or a way for them to enjoy fruit.
We did have the one where you pour the mixture in and it freezes it in a minute but we found that it didn't work most of the time.