Adobe Creative Cloud: Mac App Alternatives?
For a Photoshop "equivalent", there is already Photoshop Element (80$) that is cheaper but has much less functionalities. 

Of course there is also Gimp (Free), but IMHO it's horrible to use on OSX and not available on the Mac App Store. 

If you're looking for a simple and quite powerful software for your money, there is Pixelmator (16$) or Acorn (currently on sale for 30$).