I am not sure I agree with your calculation, but if you think the United States lacks the financial resources to maintain a conflict for extended period of time I don't know what to tell you. The fact that they are casually dedicating billions of dollars to the endeavor should inform you that financial constraints are not a concern for the US.
But it's way beyond just the billions in military aid the U.S. is providing to Ukraine, it's the effect the Ukraine war has on the EU and Russia's energy leverage.
The war is having a direct impact on EU politics because the war is creating a conflict of interest between the U.S. and EU. This is why Putin is so focused on the political aspect of the war. If EU enters into a deep recession caused by Russian gas cuts and existing problems like inflation, supply chain issues, already weak/no growth economies within the EU, etc. What do you think will happen to the relationship between EU and U.S.?
Also, despite all the talk of major Ukrainian counter offenses since March, they haven't achieved any meaningful gains in taking back lost territories while Western weapons continued to be poured in. Western arms can at most delay Russian advances and inflict additional casualties but won't fundamentally change the direction of the war. If the U.S. really wants to change they course of the war, they would need to intervene directly at this point.
