Russia and China cannot deliver more firepower per USD. Any such impression is because Russia/China only fights near-shore wars, rather than the expeditionary wars of the US.

Because the US fights global wars, it focuses on quality over quantity, as logistic costs start to dominate the production costs of military power.

If China tried to fight some Iraq war, it'll be just as expensive as the US effort. Even soldiers have to be paid far more in offensive expeditionary wars than defensive homeland wars, just to maintain morale.

One exception is mercenary groups like Wagner, they are actually extremely cost effective. That's because of much more flexible management structure, using private enterprise structures instead of rigid formal militaries, works 100 times better for chaotic low intensity warfare.