the problem is that there is natural (as in, unmodified) cheap generic insulin available, it's just that it sucks compared to everything else. you see, insulin is a peptide that is supposed to appear, do some signalling, then disappear and unmodified insulin copies this thing exactly. the problem is, most of the time when peptide is supposed to work as a pharmaceutical, you don't want to do that, you'd like insulin to last longer than usual, which means changes to it that make breakdown slower, or adding something that makes it stick to albumin, which has similar effect because it hides insulin somewhere enzymes can't reach it and also it makes it start acting slower. this means less frequent dosing and less changes in insulin activity over time. there are also other insulins that start acting faster than natural, and this is also due to a couple of modifications in its structure

for another example, ozempic was not the first drug in its class, it's also a modified peptide, and it can be injected s.c. once a week, compared to previous iteration (liraglutide) that requires daily injections. if natural peptide is injected i.m. instead, its halflife is half an hour, and in serum it's only two minutes (it gets released a bit slower than it is metabolized)

manufacturing costs are about the same for any variant, most of it is in purification. patents for a couple of these have expired anyway by now, but if manufacturing is limited then price can be set arbitrarily high (see daraprim)