To: Many possibilities, needs diagnostics and RX
She sound like a very sick pup. Glad you rescued her. Fluids and nutrition are important, but if we don't know what she has and get it properly treated she may not make it. You should be able to get basic exam / lab tests and some good treatment for way less than $2K.
I could run down all the possibilities here, but would not be helpful without knowing what you are dealing with. parvo is certainly a possibility as are parasites, infection, toxins etc. etc.
She needs to see a doctor - soon. Work with them, get estimates and tell them what you can and cannot afford.
Good luck.