Opus is supposed to be the expensive-but-quality one, while Sonnet is the cheaper one.

So if you don't want to pay the significant premium for Opus, it seems like you can just wait a few weeks till Sonnet catches up

Because Opus 4.5 was released like a month ago and state of the art, and now the significantly faster and cheaper version is already comparable.

It means price has decreased by 3 times in a few months.

Because Opus 4.5 inference is/was more expensive.