Often co-insurance is a percentage, not a dollar amount. For example one person may have 80/20 co-insurance (meaning insurance pays 80% and they pay 20%) and someone else with the same insurance may have 90/10 or 70/30. Being that different services are priced differently the co-insurance dollar amount varies for the same person a cross different services. If we could put in a percentage as co-insurance, instead of a dollar amount it would allow the actual charge to appropriately calculate. I hope this makes since.