Not only do you pay extra for a brand new iPad, older models have suddenly risen significantly in price, Heise notes. Some iPads have become nearly 18 percent more expensive this week. As far as we know, the price increase has not been announced and can only be found with some cunning internet tricks. Incidentally, the prices at retailers do not seem to have risen on the basis of our product comparator. There are currently five different variants of iPads available through the or…