Valuation under gst

Hi, need clarification on transaction value to be derived for computing GST libility to be paid by manufacturer. Green house manufacturers (poly house and net house) purchase different materials as GI Pipes/ Drip system/ Shade net etc from different vendors by paying duty. They then proceed to erect the green house for the farmer at the farmers site. Now under GST the tax rate is defined under 18 % on Greenhouse component. So basically the manufacturer will bill to farmer, including duty , takes the ITC and pay off the remaing tax liability if any. Now under the scheme for construction of Green houses, subsidy is provided by horticulture department to the tune of 50-75%, varying from state to state. I need clarification for deriving Transaction Value. As manufacture recives full amount from farmer but 65% of the amount is received by the farmer from Govt in form of subsidy. Now will this 65% amount to be excluded while calculating GST liability of Manufacturer. Regards Gaurav Saraswat