Transaction value governs supply valuation; inclusions, discount exclusions and prescribed methods determine taxable value. The value of a supply is the transaction value-the price actually paid or payable where parties are unrelated and price is the sole consideration; value includes separately charged taxes under other laws, amounts supplier is liable for but paid by recipient, incidental expenses (such as commission and packing), interest or late fees, and subsidies directly linked to the price (counted in the value of the recipient who receives the subsidy). Discounts are excluded if invoiced at or before supply or if post-supply discounts meet agreement and input tax reversal conditions. Prescribed methods apply where transaction value cannot be determined, and notified supplies may follow prescribed valuation.