which country? it would be the sale price. so if you are the retailer... buy cost - 100 vat paid - 20 total cost 120

now you sell it: sale price - 150 vat - 30 total SP - 180

while doing return, you'll do output vat - 30 input vat credit - 20 net paid to tax authority - 10 (Net of what you paid and received)

hope it helps