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.
I add frozen vegetables to a lot of things to bulk it out. Some examples include frozen spinach to pasta sauce or frozen broccoli to Rice-a-Roni-type meals.
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
Don't know about ice cream, but adding mixed frozen cauliflower to chocolate milkshake is invisible to the taste for me (and I am a picky eater) and I find it much more filling. Some spinach work too but you can add too much before noticing it by taste/smell
Even at a third, cauliflower is still invisible to me in comparison.
Try to investigate why you're paying more for homemade, because that seems odd. Are you doing per ingredient price breakdowns? Identify how you're losing out and troubleshoot that in your recipes.
When I make something more expensive than store bought, I try to make sure I know why I'm doing it. I don't trust a lot of food additives, my mindset is the price you pay with cheap food isn't in your wallet.
In most cases its just the economy of scale at work. I cant make a pizza for what Dominos can because they buy every last one of their ingredients by the tonne.
Mainly when I created this post I was thinking of cookies and peanut butter, but also presumably ice cream. Probably a McDonalds smoothie is also cheaper than a home made smoothie (if we're just looking at $/ml).
Granted cookies can be done cheaper, but when I made cookies on my own for the first time I was shocked by the butter content. Could buy a pack of low quality cookies for the cost of the butter in a 12 cookie recipe.
Iake.my own almond milk. I save the part left behind after squeezing out the liquid. Lay it out on a tray and let it dry, send it through a blender till fine. Free almond flour! I add it to my pancake mixes and baked goods for extra protein and so I don't use so much regular flour (or pancake mix).
Bananas are probably the cheapest thing you can do to stretch ice cream. Make a lot of banana splits.
Potatoes can bulk certain things out pretty well. Probably not ice cream--but you get the idea. (...probably. But maybe cooked and pureed sweet potatoes could?)