Starting to think that my cat may have diabeties, and wondering what the costs of that is? Like how much do?

How much to insulin shots usually cost? Dietary needs? Any help would be greatly be welcome. I am taking him in on the 23rd to be looked at, but he's my best friend, and I really don't want to think of losing him. He's only 8 1/2 and I love him so much. Thank for your time.