To be fair, regardless how much the customer is paying is irrelevant - If I take my car for a cheap service instead of the main dealer, and the mechanic forgets to tighten my sump plug and my engine goes pop from no oil, I wouldn't say "Oh well, I only paid X amount I can't complain".
I can't envisage myself ever offering one of these deals, but if you do it - you can't offer a great deal of 50% off your price, and then only give 50% of the service - you're only spiting the customers and in turn yourself.
If you're serious about building a customer base through satisfaction, you have to see it from their side too. Frankly, they don't care if youer only getting paid 30% of your normal rate - YOU DID THE OFFER, no-one twisted your arm.
I'm sure there will be differences of opinion, but this is how it seems to me.