gst on flat booking

hii we have some problem on gst one of builder asking me to pay 12%gst i know we have to pay 12%gst on flat booking but the issue is builder said that we have to 1500 per sq feet i bought 1350sq feet amount of 1300000 and 1.50 amenties and parking....so i have pay tax of total 1450000 is 174000 right but builder said u have to pay government rate that is 1500 per sq feet 1500×1350 is total 243000 is this right please help regarding my issue
thank you...