In reply to JayPee630:
How much is a single ticket - did you check? As others have said, a single often costs more than a return ticket. If that's the case here, they owe you nothing according to these rules that Lufthansa publish:
"10.3.1.2. if a portion of the Ticket has been used, the refund will be an amount equal to the difference between the fare paid and the applicable fare for travel between the points for which the Ticket has been used, less any reasonable service charges or cancellation fees."
Sorry to say that if so, it's not really a rip off - it only looks like one - isn't a 'rip off' where you sell someone margarine and say it's butter?