"The CD is just my voice and guitar, with another musician and/or singer on four of the 17 tracks. It sounds good to me, with consistent sound levels from track to track."

Unlike many people, I don't think mastering is essential. The fact that you've recorded your CD in a studio, with a proper mix engineer, is the main thing. Chances are your CD already sounds 5000 times better than all the CDs out there that were just recorded at home and then mastered. An awful lot of recording engineers do a quick "master" (in the sense of upping the volume levels to close to mastered levels) anyway.

Find a CD that is a comparable setup to yours (just voice and guitar etc) that you think sounds really good. Play it next to yours. Does yours still sound good to you? If so, don't bother with mastering.

If, however, you decide there is anything you think can be improved about your recording, then get it mastered. It needn't be expensive - I can recommend some people if you like.

One thing most online mastering engineers do these days is offer a free sample. If you don't like it, or think it's unnecessary, don't do it. The one thing that can be said about mastering, is that it is increasingly cheap these days.

But if you could post two tracks somewhere (soundcloud, say) to compare, I'll take a listen.

If you want to hear opinions from pro sound engineers, then post your question on the site Gearslutz.com, where all the sound engineer/mixing geeks hang oout.