@Velsy
Medical fees are expensive as hell depending on where you are. Plus it isn't like she is just sleeping. She isn't in a coma of unknown causes where by all medical accounts there is nothing wrong with her which could cause it. She had extensive brain surgery, where her chances of going into a vegetative state is 90%. So she is being actively monitored for any changes, with high end equipment. Plus the IV drip, regular tests, catheter changes, bed cleaning, room, hygiene upkeep, possibly some forced movement to prevent extreme muscular atrophy, and insurance costs. It builds up. Plus this is all way in the future in a post nuclear war and a monster invasion world. Who knows what currency rates are 1 billion could be equivalent to $100,000 USD, or only $10,000 USD.
On a side note, looking back in the first chapter, he uses a Nokia phone. Guess it would have to be Nokia who survives for over 200 years of world ending bullshit.