Listerine/ Apple Cider Vinegar (50/50 mix) - Use on everything - fungus, itchy tails, skin scruff, as a “shine”/ finish spray, clean brushes, use vinegar and water to rinse after bath for soft shiny horse
Hibiclens - The people version of chlorahexidine/ nolvasan. Not cheaper than a big gallon of it but again it works on everything - I use it to clean every wound, any fungus, add a little to water when cleaning brushes
Baby shampoo - cheap and supposedly non drying when I have to bathe my grey 5-6 days in a row at a show
Nexium - see Simkies thread re ulcers
Witch Hazel Wipes - not sure if they qualify as cheap, but these are great for wounds on areas that are difficult to clean with a hose (like the puncture wounds and cuts my mare had on her face a year ago) they are supposed to be non irritating. Have also used them to clean my mare’s udders - don’t have to worry about rinsing out soap!