On top of what Eric said (In my area, each of the smaller ones are about $10 with the giants being $15, but the triple pack is the same price), one of the ideas that was nice with Skylanders is that it allows compatibility between systems and was the only way around DLC (the Wii does not support DLC, no clue on the Wii U). Plus, you only need to use the characters once in a file to unlock the special stat boosting challenge levels. So, you don't need to buy all the figures to unlock everything if you can work up some friends to get some of the characters you don't have.
But, yeah, it is a massive money pit that will wring collectors out of house and home.