WellMixed.com offers a "cheap" mastering solution with an online interface.
Upload your file, pay a fee, and download the final mix.
Prices range from $39 for 3.5 minutes to $49 for 6 minutes of stereo audio.
A "Mixing with Mastering" option is charged $99 for 3.5 minutes and $149 for 6 minutes of stereo audio.
Examples and demos are available on the site, but only the final mix is available, not the original recording, so you can't really compare the quality before and after mixing.
Beyond the price and the concept, it's a bit ironic to see that they use IK Multimedia and iZotope products, while at the same time advising you to "Avoid the Do It Yourself Sound"...