I like adding things to my icecream, usually peanut butter and frozen fruit. Got to thinking that if I added oats I could actually increase the volume without impacting the flavour all that much (I like oats). I could probably use floured starches or something like that.
Are there other things you “fill”? I think juice + water is the most familiar example. What about something like adding 20% dehydrated milk to fresh milk? Substituting some butter for oil?
Sometimes I find when I’m making my own stuff it ends up being more expensive than buying the packaged variety from the store, but maybe fillers are a way to balance that out.
Dried foods are the best value, I would experiment with that. Rice is famously filling and cheap
In the same area, beans and lentils can be added to everything to stretch it out including the aforementioned rice lol. I feel like everyone knows this by now though. Frugal staples are dried bulk rice and beans
Dried mushrooms are also significantly cheaper than fresh.