Coinsurance is one of the least-understood health plan costs. What you don’t understand can take a bite out of your budget. Coinsurance is the percentage of medical bills you have to pay after you’ve met your deductible. For example, if your endo pain sends you to the emergency room and the bill is $10,000, with 20% coinsurance, you’ll have to pay $2,000. Understand that percentage so you can prepare for those potential expenses.