Hi, if you are on Mac, probably the best choice is Pixelmator. Current price (during May) is just $14.99. You can't find better editor for that price. You can win a free license in our monthly contests
As for the Windows, it's hard to say. Many people would recommend GIMP, but I personally consider it a waste of time, even if it's for free. If you are about to work with RAW files, the best value for money is probably Lightroom. But my personal favorite is still Silkypix.