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.