They're out for a long time, but they shift to glowie balloons in the evening. Since you cannot see a balloon during night, they tend to switch balloons in general out for glow-in-the-dark trinkets. The shelled balloons cost at least $12.
If you are searching for souvenirs, in my opinion, this is probably a pretty low value. It's tough to travel with on a plane (if you can even get it through security) and has little lasting effect unless you get some photos with it, in which case you would need to get it during the day and not the evening (it sounds like you are looking to get one later on? I am inferring from your question), which means now you have to figure out what to do with the balloon during Splash Mountain and whatnot. If you want a balloon, by all means get one. You know better than anyone what you and your family will value. But do consider doing something else. You can always pick up some Mickey balloons for $2 from Party City and bring them with, then sneak in to the room while everyone else is out and add them as decorations. Much cheaper and you won't have to worry about what to do with them.