There a few reasons why your cat has this problem on the tail.
Could be to do with chemicals in your home like xxtiffiexx says,
a change of food, mite problem, dermatitis, excessive licking.
See if you can find DermOpt which is available without a prescription. There have a range of skin and coat care products for cats. You can check them out on the web.
Best wishes.
im sorry to hear of your kitty's troubles....I will send you a site to do more reading on this..
you said you cannot afford any testing or a Vet visit, than I would strongly suggest you try to eliminate all chemicals in your home, especially room sprays or anything perfumed...that can sometimes be a factor.
If you are feeding dry food can you try to buy cans....sometimes the preservatives in the dry food causes food intolerance's, so does corn and other grains...grain free can food is a little more expensive, do you have a pet store nearby you can shop at and not a grocery store? they usually carry the better grain free diets...it would be helpful if you could afford to try these.
good luck, I know you love your kitty and you are trying.
http://www.cat-health-guide.org/cat-losing-hair.html