Claude Sonnet 5 is more expensive (around +15%) per task than Opus 4.8 and much more expensive (2X) than Sonnet 4.6, even though its per-token price is lower than Opus.
Because it uses more tokens to complete the same kind of benchmark task. i.e. Sonnet 5 works harder and talks/thinks more, so the final bill becomes bigger even though each token is cheaper.
The promo pricing changes the story for now. Until August 31, 2026, Sonnet 5 is discounted to $2 per 1M input tokens and $10 per 1M output tokens, then it moves back to $3/$15 from September 1, 2026.