Export Block Cost Estimates

Aggregate per-block average credits-per-execution over [start, end].

Capped at ANALYTICS_MAX_DAYS days. Returns only blocks whose current cost type is dynamic (SECOND/ITEMS/COST_USD) — static-cost blocks already charge correctly pre-flight and don't need an estimate override. TOKENS is excluded because compute_token_credits already supplies a per-model floor at pre-flight; a per-block historical mean would lose that granularity.