How much can you realistically make a weak/how easy is it? I'd say my wife gets through around 2 litres a week, which at £3.50 or so isn't too expensive.
How much cheaper are you talking?
Throw a handful of rolled oats and some water into a blender. Blend. You have oat milk I chuck a bit of coconut oil, honey, bit of salt in. Nuts like cashews are supposed to be good for making the creaminess like the Barista versions.
There's a variety of additives/stabilisers/sweeteners in the commercial stuff, but fundamentally it's mega cheap to make. Certainly compared to rearing cows and squeezing milk out of em, it's mad that oat milk is more expensive!
Oatly must be making bank on their products.