You get what you pay for in most instances.
NOTE..... what is being overlooked in this discussion is that you don't generally "master" a single song. Mastering as I understand it is a process done to multiple songs which are all slated to go onto a singe CD project so that they all sound similar with respects to levels, compression and EQ. For clarification.... when I used the word "mastering" here, I'm referring to small "m" mastering which isn't really mastering at all but is simply what I call "sweetening the mix".
I saw somewhere, someone posted a link to some sort of automated "mastering" on the net for free... "Guaranteed to make your song sound professional".... so out of curiosity, I submitted a tune, raw from the DAW with my plugs shut off. The so-called mastering was completed in mere seconds and the MP3 was available for me to hear. It was not even close to sounding as good as my work.
With a basic Ozone pre-set and no tweeking, you can get as close or closer than some on the folks who think they're mastering engineers on the internet.
Assuming $30 a song.... and a reasonable base rate of $60/hr intended income level.... that gives them 30 minutes total to work on mastering your song. That time includes downloading the song, loading it into the DAW, applying the mastering effects, perhaps 3 to 4 plays of the song to tweek the FX, and exporting it and uploading the results to you. That's a lot of stuff to do in 30 minutes. Folks who are really good at "mastering" could probably do it in that short time span, but those guys are being paid more than that to work in a professional mastering house.
A more reasonable expectation would be $120 for mastering with as much time as is needed to do the job right. It's possible to get a decent job done in under 2 hrs total. If I was planning on going into that business, I would factor a minimum of at least 1 hour to start with and have contingencies for overtime in half hr increments. Look up the album mastering prices at the professional mastering houses.
Price list from a decent mastering house.... not the best in the business, but good:
http://www.discmakers.com/soundlab/pricing.asp