I have found that I do not need extra medical insurance in Cuba. I have never been asked for proof of insurance when entering Cuba (and that's many times). When applying for a Visa Familial or 90-day visa extension, my OHIP card has good enough for Immigration, and I didn't need to buy their expensive insurance.
Another contributor to this site stated that he had medical work done there. He kept the recepits, showed the exchange rate for that day, and was fully reimbursed.