Why are there such huge differences between commissions on different exchanges?
It varies from exchange to exchange because people running exchange are different with different thinking. Me and my brother runs a website where we sell coins in exchange of USD(we are not a exchange anyway just small dealer) we do charge some fees but the difference here is my brother will charge a customer upto 20% higher than actual rate on the other hand(u have different fingers) I charge only that amount which is reasonable.