Betadine! It's cheap just dilute in a spray bottle with water to look like a weak tea. If you don't want it to stain just scrap the scabs off and then give them a bath with it just dilute the same way in a bucket and put it on with a sponge leave for about 30 seconds then rinse clean. Super effective and cheap
I bought some stuff from Dover it was the Absorbine fungasole ointment and it is AMAZING! I used it 3 times and my horses rain rot is almost complelety gone! I definentaly recommend it and the price is great!