I'm kind of a princess when it comes to my art software/equipment.
I use:
Photoshop CS5 (I actually have the whole Adobe Creative Suite and I upgrade it with every new addition. Apart from Photoshop, the Adobe program I use the most is Illustrator.)
Cost: Well since I'm a student, I get it at a very serious discount, but it's still a few hundred dollars.
What I use it for: Photo editing, photomanipulating, doing special effects on paintings, and doing rough sketches for paintings.
Is it worth it: It is to me. I would be lost without Photoshop.
Corel Painter 11
Cost: Again, it's a few hundred dollars at the student price.
What I use it for: Painting, drawing, and lineart.
Is it worth it: YES. I've never used another program that had the capabilities of Painter and that was as nice to use.
Extra Large Intuos4 Tablet
Cost: Mine was a gift, but they run for about $800
What I use it for: Everything. I can't draw with a mouse, much less a track pad on my laptop.
Is it worth it: YES. Having a tablet, especially one that is the right size for your computer screen, makes your life as an artist SO much easier. If I hadn't been given one, I would have bought it for myself.
I also work on a Mac. I have a 17" Macbook Pro laptop that I got specifically for it's art capabilities. I got it back in 2008 and have asked it to do all sorts of ridiculous things and it still runs like it's brand new.
XD I told you, I'm a princess.