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No, you are misunderstanding the graph. Draw a vertical line anywhere, that is a "constant cost" line. For any given cost, Opus 4.8 has a higher performance than Sonnet 5. Only where Sonnet 5 effort is at medium or low would it make any sense to use it, as there isn't even an equivalent Opus effort level to compare to.

Alternatively you can draw a horizontal "constant performance" line and see that Opus is cheaper for a given performance level.

Why are you comparing xhigh reasoning between Sonnet and Opus? Of course Sonnet xhigh is cheaper than Opus xhigh, but that isn't the point; the point is that at e.g. 80% accuracy on Opus costs ~$0.45 (medium reasoning) whereas on Sonnet it costs ~$0.52 (xhigh/max reasoning).

That is a bad comparison. Compare Sonnet xhigh against Opus medium, which is both better and cheaper.

No, that's apples and oranges. You need to compare Sonnet5's 79% with the interpolated Opus4.8's 79%.