What sort of illness does my cat have?

You expect one of us to magically diagnose your cat?? Get it to the vet. No one online can tell you what's wrong or what it will cost to treat.

edit: $50 may cover just walking into the vet and that's it. If you can't afford this cat then surrender it to a no-kill shelter. Did you meant to say your parents WON'T take her? If so - that's considered felony animal neglect in most areas. And if the cat is living in their home THEY are responsible for it's well-being - not you. And thanks for the nasty email - I forwarded it to Yahoo.

Has the fur plucking just started since you left home for school? She could be stressed out that you aren't at home. 1 of our cats did that when we left on vacation and again when he was ill and was at the vet for week even though we visited him 2x a day.

If it has been on going it could be an allergy that the vet could give her a depo medrol injection and that is not very expensive at all.

Could be food allergy or fleas?

I would guess th cost would be an office call, possible skin scraping to check for fungal disease and a maybe a blood test for allergies. And a antihistamine shot or pills.

Maybe $125 all in. Just call the vet to get a base price on visit, some give student discounts, elderly discounts.

Ask a vet, or if you're worried about money go to the humane society and work out payment methods. It's your cat, man. Don't let some internet idiot waste your time and keep your pet from health.