Transaction value governs taxable supply, with specified inclusions and prescribed discount conditions determining GST valuation. The value of supply is primarily the transaction value-the price actually paid or payable when parties are not related and price is sole consideration. Taxable value must include specified additions (non GST taxes charged separately, amounts borne by recipient but payable by supplier, incidental expenses, interest/late charges, and subsidies linked to price). Discounts are excluded only if invoiced at or before supply or subsequently established by agreement with reversal of related input tax credit. Prescribed methods apply where transaction value cannot be determined, and the Government may notify supplies for special valuation. 'Related persons' are broadly defined.