If we take a look at the % of US forces stationed in Europe, and assume all of them are there only out of the goodness of the US' heart to protect Europeans, and subtract them from total US military costs, it isn't even a drop in the bucket. It's less money than the Pentagon doesn't know what happens to.

https://www.defense.gov/News/Releases/Release/article/307805...

(This only covers the running costs; even an unfair assumption that the US would need less F-35s and troops if they weren't deployed in Europe, it would still be a drop in the bucket compared to the $916 billion yearly budget).