GPT-6 Astra standard pricing is $10 per million input tokens and $50 per million output tokens, with cached input at $1 and cache writes at $12.50, per OpenAI's model page.
A prompt over 272,000 input tokens moves the whole request to long-context pricing: $20 input, $2 cached input, $25 cache writes, and $75 output per million.
Claude Opus 5 at $5 input and $25 output has no context-size surcharge, so it is often cheaper than GPT-6 Astra for large prompts despite a smaller headline gap.