Function estimate_saved_cost 

pub fn estimate_saved_cost(
    tokens_saved: u64,
    a: &SavingsAssumptions,
) -> SavedCostEstimate

Estimate the billed cost avoided by tokens_saved under a prompt-cached serving model. This deliberately models the caching-enabled case, which is the conservative one: without caching every re-read bills at full input price and the figure would be much larger. Direct token cost only — trajectory effects (extra agent turns caused by missing detail) are not modeled here; the regret metrics in sqz stats exist to watch for those.