It's worse to provide incorrect information than no information at all. I understand that they're dependent on what their pricing partner provides to them through the API, but if they can't get accurate pricing, they could mark the values as unknown, while retaining the clickthrough to the TCG site. Could also implement a table filter to allow you to see the subset of your cards that are promo/textless/foil/signed/etc. and therefore might have differing values.
There's typically a temptation to throw up your hands and give up when working with a third party that doesn't provide you the calls you need, but there's usually something you can do to make sure your users aren't led into making mistakes. And if it was a dealbreaker for using the tool, why would we be coming on the forum to suggest improvements?