I couldn't afford mastering, but I'm very happy with the way my CD turned out. One of the things mastering does is makes sure the volume levels are similar in all the songs, so the listener doesn't have to keep adjusting the volume, and it's also a general tweaking and polishing done by another set of trained ears. If it sounds good to you (and you need to play it on a variety of sound systems to make sure) you are probably good to go. I think mastering is more important when you've got complex mixes, effects, lots of percussion, heavy bass, etc.   I'm sure there will be other more informed comments than mine on this topic.