How much additional cost does it actually cause the US, though? It's not like the DoD or the intelligence community would drastically downsize their budget or their staffing levels if the US pulled out of Ukraine entirely. You'd have more intelligence operatives or logistics units sitting idle or go underutilized, but they obviously wouldn't be cut. Essentially, at the current level of engagement, the US is mostly putting the excess capacities of its bloated military and intelligence apparatus to actual work.
I'm not sure that's the way it works. US has dozens of European bases that primarily exist and have existed to counter USSR then Russia, these are ongoing spending commitments in perpetuity. There's a bunch soldiers, infrastructure, intelligence workers, spy planes, satellites and on and future commitments for this. If you moved many of those assets to lets say ME or east Asia to counter China, IMO sure some of the buildings/physical infrastructure may be under utilized but i bet the vast majority can absolutely be redirected.
Trumps 50% ask for DOD increase next year i think is in part because they don't want to move significant assets from one theater to another. Don't you recall Ukraine complaining about AA redirection from them to ME? They don't want to cannibalize the European defense infrastructure, so only solutions are 1. ask euros to spend more and 2. increase spending for DOD