Value of supply determination under GST: prescribed valuation rules govern cases where consideration is not solely monetary. Section 15(4) directs that where the value cannot be determined under section 15(1), the value shall be fixed in the manner prescribed by valuation rules. The rules provide specific methods for situations including consideration not solely in money, supplies between distinct or related persons, supplies made or received through an agent, and other specified cases. Where price is not the sole consideration, the open market value is used to determine the value of the supply.