I have always thought exactly the same think. Makes no sense at all. I always say 'cheap at twice the price' instead.
Do you think it may be a corrupted phrase and it was originally 'As it is half price, it is cheap? But it is never used in that text, it is always used when referring to the price presently being asked of anything, weird phrase.[/u]
Yes, did wonder if it was used differently long ago, but has been corrupted more recently. Still can't make a lot of sense of it either way.