The canonical megaprojects had relatively modest budgets

The Manhattan Project began modestly in 1939, but grew to employ more than 130,000 people and cost nearly US$2 billion (equivalent to about $23 billion in 2020).[1] Over 90 percent of the cost was for building factories and to produce fissile material, with less than 10 percent for development and production of the weapons. Research and production took place at more than thirty sites across the United States, the United Kingdom, and Canada.

Of this amount, $20.2 billion ($126 billion adjusted) was spent on the design, development, and production of the Saturn family of launch vehicles, the Apollo spacecraft, spacesuits, scientific experiments, and mission operations. The cost of constructing and operating Apollo-related ground facilities, such as the NASA human spaceflight centers and the global tracking and data acquisition network, added an additional $5.2 billion ($32.3 billion adjusted). [wikipedia]

Apollo cost $25.4 billion (or approximately $158 billion in 2020 dollars when adjusted for inflation via the GDP deflator index).[1]

Project Apollo was a massive undertaking, representing the largest research and development project in peacetime. At its peak, it employed over 400,000 employees and contractors around the country and accounted for more than half of NASA's total spending in the 1960s.[136] After the first Moon landing, public and political interest waned, including that of President Nixon, who wanted to rein in federal spending. [wikipedia]