Here's a great article on how to get low cost alternatives to Adobe Software for artists on a budget by Geoffrey Goetz
Here's an excerpt:
Through apps mostly available via the Mac App Store, you can decide
which components of the software set matter most to you and build up
your own custom suite over time to meet your creative needs. Here are
the best alternatives I’ve found that offer similar functionality to
what is available in each of Adobe’s products.
Photoshop to Pixelmator ($14.99, Mac) By now it’s no secret that Pixelmator is one of the favorite apps on OS X for many.
When you think of Photoshop, you think more than just applying filters
to your images; you want to edit them. With its multilayered support,
you can easily touch up and enhance your images with Pixelmator. Many
of the same tools that Photoshop users have become accustomed too are
here too, like the smudge, sponge and brush tools. Pixelmator also
comes with some basic vector tools that may be all that you need from a
full-featured vector editing tool like Illustrator. For most of your
day-to-day image editing needs, and at the current low price point of
$15, it is hard to beat Pixelmator.