Contract price is more specifically referred to as ‘transaction value’ and that is the basis for computing tax. However, the transaction will not be accepted as the value of supply where the supply is between related persons (including different registrations of the same PAN and principal-agent supplies), or where the consideration payable is not wholly in money.
(ICAI FAQ PUBLICATION 06-09-2017 : Time and Value of Supply -FAQ NO. 43)