The government has people dedicated to figuring out this stuff. The CBO. They advise Congress and publish reports on legislation.
Every time the Republicans say these things will pay for themselves, and every time the CBO is like, “uh, no, it’ll cost $X trillion over ten years”.
Everyone ignores them. Now we’re several trillion deep in debt from George W Bush and Trump tax cuts, as predicted by the people who actually know how shit works.