I have no idea how _much_ money they're losing because the images are on Wikimedia, but I think it's reasonable to assume there's a noticeable loss.
My guess would be they're not experiencing a loss at all. If their revenue stream is based on prints from their gift shop, or even exclusive'deals with online print sellers, I can't see someone not buying from them thinking, "Oho, I can just go to Wikipedia and print it out on my color printer for free!" Hrm, or maybe it's other online print sellers grabbing the wiki entries and selling them that's the problem?
"somebody has invested money, skills and effort in making the reproduction be as good as possible."
True, a lot of sweat went into it, but does that make it an original work of art which should be afforded the same protections as a any other piece under the law? Apparently under UK law, yes; fairly dumb if ya ask me.
A penny saved is a penny to squander. -- Ambrose Bierce