Where a contract defines 'actual cost' by reference to a specific formula (multiplier x raw rate), the formula governs the party's entitlement to reimbursement, not the ordinary meaning of 'actual cost'. An auditor's functions under such a contract do not extend to open-ended investigation of actual costs determined without reference to the contractual formula. Silence and acquiescence in the face of progress claims made on a particular basis, combined with payment of those claims, may give rise to an estoppel preventing a party from later asserting a different basis of entitlement.
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