That doesn't sound right. I get a volume of about 1.13 cubic cm per 100 dollar bill. There are 440 million 100 dollar bills needed for 44 billion. That gives us a volume of about 500 cubic meters. That's not even a large warehouse. Even for 1 dollar bills we would then only have roughly 50000 cubic meters, which is a far cry from 500 cubic kilometers, which would be about 5*10^11 cubic meters. A single stack of 44 billion 1 dollar bills would be about 4800km high.