What's the deal with these? Do they get more money the longer you hold on to them?

It's a random amount, normally not worth opening it.

You would actually get more money if you sold the item instead of using it! But yeah, unless they've changed the sP amount alongside the art revamp, it's just a gimmicky item. Unless they added an achievement for using piggy banks.... ... ...Ho jeez, start stocking up on piggy banks! :O

Most of the ones I have opened were small amounts but I have opened one that was pretty good. That was a good question. Now I am wondering if they might increase the longer you have them.