The saying 'cheap at half the price' apparently means a bargain...surely 'cheap at twice the price' would make more sense, ie something you really want, you're prepared to pay twice as much!?
The saying is 'cheap at twice the price'. 'Cheap at half the price' is a humorous version which people sometimes say as an alternative, the point being that it sounds like the first though its meaning is quite different. That version is not meant to be taken literally.
actually, I suspect most people who say it are just getting it wrong and weren't meaning to joke at all. Like saying 'having your cake and eating it'... anyone can do that, it's eating your cake and still having it that's tricky, but it gets quoted wrongly all the time.