Contact your local ASPCA or
attempt to find an animal rescue organization in your area. They may be able to help subsidize the health costs. An antibiotic of choice such as Cephalexin isn't that expensive. However as someone else pointed out the diseased tissue will have to be surgically removed, and would incur Veterinary charges.
Don't attempt to doctor it yourself. I'm sure if you visit some "natural healing" forums suggestions will include things like peroxide or iodine which will just make things worse.
Be persistent in finding help. The infection isn't going away on its own.