A common forgotten point is that they spend that money partially because they had to it else they could not have guaranteed that Europe would not align with communism. At the same time doing so saved the USA from a economical crash caused by overproduction. Lastly it allowed them to gain deep tried economical influence.

While the USA might sometimes to present it as a selfless act it wasn't selfless at all. It was what allowed them to become the most powerful country in the word, at least for a view years.

Lastly Trump didn't just want to buy right on the potential vaccines he wanted to get exclusive rights. It was never even a question before that if the EU finds a vaccine then the USA will get it, too. But now he did an action which completely destroy the trust between the EU and the USA when it comes to fighting this pandemic together.