You don't necessarily have to pick something from Nintendo's list. They only recommend the ones that they do because they're self-powered. But they do state at the bottom that they don't guarantee any of those will actually work, aaaand those also tend to be a bit more expensive.
If you pick up a Y cable -
http://www.amazon.com/eForCity-dual-Micro-B-Cable-Black/dp/B005M0ICG2 - you won't need to worry about that. The Wii U just isn't powerful enough to run a hdd off of one usb port, so it needs to be powered by two of them. And then you can go on to pick out a cheaper option. Such as either of these:
http://www.amazon.com/gp/product/B0...edit-tech-round-up-20&linkId=YQR5372NSEO2HH43
http://www.newegg.com/Product/Product.aspx?Item=N82E16822236517
(This one is cheaper on newegg, but an Amazon review was written for the Wii U, if you're interested:
http://www.amazon.com/Elements-Portable-Drive-Storage-WDBUZG0010BBK-NESN/dp/B00CI3BLPA )
If you would feel safer getting one from the list, the MyBook from WD is available, and the price isn't too scary. =p
http://www.amazon.com/Book-External-Drive-Storage-Backup/dp/B0041OSQB6