There are viable options for a subset of photoshop's functionality. As you note, you can get 80% of the functionality for a huge difference in price, which is fine if you don't need that last 20% for whatever reason.

You can try pixelmator(1) (mac only) or pixel(2) (cross platform).

1) http://www.pixelmator.com/

2) http://www.kanzelsberger.com/pixel/?page_id=12

Take a look at Acorn (http://www.flyingmeat.com/acorn/) by Flying Meat. It lacks some of the more esoteric PS/Gimp features, but overall is substantially easier to use and has all your critical features (layers, filters, color adjustment, etc.). Oh, yeah, and it's cheap, too (free demo, $50 to buy)...

For designing the layout: http://en.wikipedia.org/wiki/Pen http://en.wikipedia.org/wiki/Graph_paper

For working with images: http://flyingmeat.com/acorn/

I have yet to find a comparable alternative to Photoshop. There are many attempts at duplicating it, but none of them are nearly as powerful as Photoshop.

I believe there are some "lite" versions of photoshop out there for cheaper prices, though...

Gimp?