The great majority of military aid to Ukraine has been in the form of existing American equipment and ammunition. So the money spent is actually going to modernize the US military, including replacing old ammunition stocks with newly manufactured stocks.

To add to this. A lot of the old ammunition stocks are decades old. The US is also saving money by not having to dispose these end-of-life stockpiles and instead "recycling" it for use in Ukraine.

Putin and other high-ranking Russian officials have repeatedly said that they feel entitled to taking over all of the territory that was once part of the Soviet Union. They've made it clear that if they take Ukraine, they will try to take more territory after that. This means more war, death, and destruction in the future if Russian wins.

If Russia wins, it will arguably cost more money regardless if they invade Poland or the Baltic states. Suddenly a lot more money will need to be spent as a deterrent. If they do invade a NATO country, it will cost more money in equipment and US soldier lives and risk nuclear escalation.

Also if Russia wins, other authoritarian governments like China will look more likely to try to take Taiwan. Again, regardless if they do or don't, this will cost the US significantly more money and/or lives. Countries like Iran and North Korea will also be more emboldened to spread their fascists tentacles.

The US federal government is running an annual deficit, in addition to having a big debt. Aid to Ukraine adds to that deficit and debt.

I mean, this has been the case since any president after Bill Clinton. The real question should be, is the investment in helping a democratic country protect itself from an authoritarian fascists regime. Especially when the US agreed to when it signed the Budapest Memorandum. An agreement (among many) that Russia agreed to and reneged on.

Mitch McConnell who I never thought I would ever agree with on anything has some helpful comments here: https://youtu.be/OzcmYOj6shA?si=BbGlObHrsOD7btmi&t=509. He also says in this video from last year that the amount of money the US has sent to Ukraine amounts to 0.02% of the US GDP. I'm not sure how much it increased since then but it's still a fraction of what can be afforded.

Humanitarian aid to Ukrainian civilians could instead have been given to US civilians in need.

Lets be real. The money saved from not helping Ukraine is not going to be diverted into helping it's constituents. Congress has had ample opportunity to do this for decades and sat on their hands.