I think one can infer that, by pricing 2x between gpt-5.4 and gpt-5.5, the delivery of gains within a month was likely by a 2x test-time compute, technology like “pro” does with 12x the billing - and where the pro per-token bill was not increased between these version steps.
So I would guess we have similar (gpt-5.4->gpt-5.5) === (gpt-5.6 terra->gpt-5.6 sol) - at least as your pricing guidance.
Notable is API pro has no context window cache discount, so that can make costs of tool iterating extreme.