I didn't have budgets handy, but GDP is another way to normalise its cost to the country; ~$2.2B cost in 1942 with a nominal GDP of $166B, nominal GDP of $29.2T in 2024 puts the Manhattan Project's cost relative to GDP at $386B in 2024.

Nominal GDP from https://en.wikipedia.org/wiki/Economic_statistics_of_the_Uni...