I have heard using Rubbing Alcohol works on killing the ring worm. Maybe even Petroleum jelly. Make sure it is applied with rubber gloves!!! It is HIGHLY contagious. There may be some over the counter options.
Ring worm is transferred through the nose or feet/paws. The dog smell the infected "poo" and it goes up their noses. So make sure they don't "sniff" poo if possible. I make my dog's avoid piles when on leashes.
My dogs got infected and the treatment wasn't that expensive. Some medicines are applied into the nasal cavity directly and others are oral. I had 4 dogs that had to be treated. It took about a month of medication taken once a week. I think it was less than $150 for all of them including the vet visit. The doctor gave me enough to treat all 4 even though 1 was diagnosed. Good luck!