It's actually not a bad idea, the price may need looking at, but it is a lot better than Groupon. For a start-up business that is looking to increase its customer base it could be a good thing. Better still if the initial deal was sold as an introductory offer at 50% off the normal rate, that way the customer knows next time it will be more but have at least had the opportunity to check out your work to see if you're worth more, if so, they'll probably ring you back.
Simon