Why is my cat scratching herself?

i have a three yr old balinese, the last month she has been scratching herself to the point of blood, she has no fleas ticks etc. i took her to the vet, for them to charge me $150 to tell me it may be dry skin and to give it a few days, i told the vet she been doing this for what was about 2 weeks at the time. i figured id wait and see if she stopped but she hasnt, shes been tested for feline aids and what not and everything think came back good. what can i do to stop her from scratching, i tried putting her in one of those neck tubes and she just goes crazy clawing everything in site. someone told me to try putting the aloe gel stuff you put on sunburns but idk about that, seems kind of sketchy. any idea?