What a mess your friend dumped in your lap. You're a dear to go to these lengths for a snake you didn't plan on owning...good for you!
When you do contact a vet, please mention that this is a rescue snake that came to you basically homeless and sick. Many vets will not only work out a reasonable payment plan but some vet clinics actually have funding they quietly access for these kinds of situations. Also some vets offer discounts for rescuers, especially with exotics since so many of these creatures have no where to go when they become unwanted pets. Never hurts to ask.
Please let us know how things go with this poor snake. I think you are doing a wonderful thing and certainly teaching your children some great life lessons in the process.