You can define non-price for 3 kingdoms using map_index.
Search for if (pkChr->GetEmpire() != pkChrShopKeeper->GetEmpire()) in shop_manager.cpp and add this bellow
// this method is only for cape by pirulipipi
if (pkChr->GetMapIndex() == 211)
bOtherEmpire = false;
211 its map index where prices will be x1 for 3 kingdoms