Whats an EASY and CHEAP digital art software for mac?

Gimp?

I would say the leaders are GIMP, for a bitmap based computer drawing software.

Inkscape for vector graphics.

Artrage for a programme that emulates non-computer art media.

The first two are free. Artrage is $50 for the full version, but only $30 for the lite version, and there is also a free demo which is a lot of fun, though not so full featured, and not time limited.

Of the three Artrage is the easiest to pick up and use.

If you want a more computer-type drawing programme that is accessible you could also look at the drawing/presentation package within an office software suite. When I first used a Mac it came with the ClarisWorks suite (long time ago) and ClarisDraw was really good. I think it actually got less good in later versions, bizarrely. But still quite useful when it became Appleworks. I don't know if that is still the case with the current bundled programme on macs though. Tthe drawing and page layout capabilites of MSOffice are certainly totally rubbish. Unfortunately you can't get a download of Clarisworks because Apple own the copyright and won't release it, even though they've abandoned it. Fortunately OpenOffice, which you can download for free is pretty similar and might be useful to you.

Use a free online editor like Pixlr.com.
A $30 app wont' be worth what you paid for it.

GIMP is free. It works just like Photoshop.