Is it as simple as you say? That she literally doesn't know the price of a single, popular item after 6 months, or is that just a very simplistic example? If it really is that bad, then you've obviously got to let her go. But (and I have worked in shops!) there is nothing more annoying than items appearing with no price on them (without warning and without an EPOS till!)
You've obviously got to do the right thing for your business, but spare a thought for her and her future - could you have done anything differently? And how will she fare in future employment having been sacked (albeit for not knowing the price of a mars bar!)