Some time back (pre Iran war and associated issues), I was listening to a military analyst being interviewed and apparently it is cheaper for the US to barrack troops in Europe than have them based in the US. Main reason is because Europe provides bases, land, accommodation, etc. for free.
So if US withdraws its troops back to be based in US it will cost them more US$ (and save Europe €s/£s).
Simiularly, US is not spending more or having higher military budgets because of NATO. It's spending that amount anyway except as above for their saving on barracks for overseas based troops). If US left NATO they would still have the same military budgets/expenditure.