I think we are buying lots of covers from Europe (more than 25 only from an Spanish forum in less than three weeks) so they are trying to take advantage of our interest and have risen the prize 10$. Awful, but no more orders from Spain right now (from the forum, I mean).
Another thought: If they realize (and are going to) that the cover fits the new model PRS-600 like a glove, the prize could be restored to the original 39.99$ as there is no need for an offer.