Many companies manufacture insulin. There's also not just one kind. Two pervasive types are fast acting and slow acting, the latter typically used to provide a low level continuous boost throughout a day. Manufacturing high quality product in volume isn't particularly easy because it's a biologic drug requiring genetically modified organisms to create the protein, typically E Coli, and purification to human injectable standard is costly, requiring removal of the pieces of cells that were used to make it, including endotoxins in the case of E Coli.

Beyond that, there's an easy way to guide analysis of what appear to be excessive costs in any market: find out where government force has screwed it up. It will always be the cause.