Any radar that can see F-22 or F-35 will in turn be visible by them. It will be a cat-and-mouse game. In which case training, doctrine, improvisation and quick learning matter a lot. Weapons may be expensive, but training is expensive too. Expenses aside, how can Russia train to use its S-500 against US stealth jets? Using what jets as mock F-35's? Conversely, the US can very easily train against a simulated S-500 battery. It's not like S-500's radars are some big secret.
I think you are missing one of the key ingredients to anti-stealth radar, and why we have shifted to a strong focus on RAM. That would help answer your own questions. I won’t say it here, but separately it certainly is possible to train against targets which would produce returns analogous to an F-22 or F-35 type aircraft to various radars.
On another note, US anti-air capabilities are already far behind the S-400. I understand we don’t have an immediate use case ourselves, but the lack of a credible AA system pushes potential partners like India and Turkey to fight with us for the right to buy the S-400.
