As I write this in 2019, census.gov estimates the USA’s population to be 329,408,944.
My mental shortcut is that that number is roughly 1/3 of a billion. Every time a politician says $1 billion, I think of it as “$3 for every adult and child in America”.
Example: NASA’s annual budget is $21 billion. Just multiply that number (without the “billion”) by 3 to get the dollars per American, large and small. 21 times 3 yields $63 per person. Each year.
Second example: The National Endowment for the Arts has an annual budget of $150 million, which sounds like a lot. But if you convert it into $0.150 billion and multiply the number times 3, you find out it costs each person $0.45 or 45 cents each year.