The bizarre bit is that it's pretty expensive to gain a new customer, and cheap to keep an existing one, so you want to make it really easy for people to stay with you. A loyalty discount would be the way to go, rather than a loyalty over-charge. Supermarkets and cafes with their loyalty cards understand that.
My previous employee, a mobile operator who shall be nameless but involved a magic number, estimated the cost to gain a new customer was a £200 overhead (initial call/walk-in, account set up, set up phone, sending SIM out, first contract letter, initial support, etc) - which was maybe 9 months of an average 24 month contract.
Having said that, their customer service leaves ever such a lot to be desired, so I know where they've tried to claw the £200 back.
