Depends on the type of insulin you want to make, but if you want to be at the cutting edge it would likely cost around a billion dollars.
You have to avoid the intellectual property minefield on the manufacturing side. The production of insulin and its delivery mechanisms are heavily patented and the big pharma companies will shut you down if you infringe on anything. If you want a reference for this happening you can google 'lusduna'.
Plus you have to make it through the FDA, which is its own can of worms. The 505(b)(2) is still not a cakewalk but at least it will give you 3-5 years of market exclusivity so you can recoup your costs. (Wait you can get market exclusivity for a biosimilar with an expired patent? That should sound insane to anyone not in healthcare.)
Then you have to actually manufacture and distribute your product. There are extremely high capital costs to get set up. For reference, Novo Nordisk's insulin manufacturing facility in North Carolina is 328,000 sqft and staffed by ~500 people 24/7. Just to get a comparable facility up and running you're talking about 400-500 million dollars in capital cost. Sanofi, Eli Lily, and Novo Nordisk have spent billions over the last decade upgrading and maintaining their facilities.
I would suspect it's about the hardest project you could tackle if you were a startup.