Retail sale price determination: highest ascertained price governs when declaration is missing or package price is tampered. Rule 12 sets a hierarchical procedure for determining the retail sale price when declared prices are absent or unreliable: adopt the declared price of identical goods removed within a proximate period if available; otherwise, ascertain price by sample-based retail market enquiries where such goods are normally sold; apply the highest price if multiple prices arise; treat post-manufacture increases due to tampering as the retail price for goods removed within a temporal proximity window; if none apply, determine price by the rule's principles.