Summarize is 3 credits for documents up to 100 pages. Above that, we charge 1 extra credit per additional 50 pages because the model has to fan out across multiple chunks and then re-combine.
Tables and figures also cost slightly more. A 60-page financial statement with 30 tables can come in at 4–5 credits even though the page count is small, because each table is parsed separately for accuracy.
If you want a strict ceiling, run the Compress tool first to drop image weight, then Summarize. The page count stays the same but the tool runs faster and never accidentally tips into the next tier.