Long story short, $600 is a totally fair price for a one-off mix from a guy with ample experience and a good-sounding reel.
If anything, $600 is fairly inexpensive for a really decent mix of a single song at a good studio. $400 would be a real bargain. I don't know many people who I'd trust to do a good job who'd mix at a rate less than $400/day. And remember, a relatively dense modern track could easily take 8 or more hours to mix. especially if it's a one-off.
What you're really paying for is time and expertise. In my experience, a decent studio with a proven engineer crafting a compelling mix will likely run you $400/day at the low-end of the spectrum, $800/day and up or so for someone you've actually heard of (who's mixed a bunch of records you've actually enjoyed listening to) and $1,000+ for someone very much in demand. Things go much higher than that if we include the exclusive cabal of major-label mixer-dudes and successful snakeoil salesmen.
Of course, if you're lucky and in an inexpensive market, you may be able to find someone who's fairly capable but still learning for something more like $200-300/day. College students itching for experience may charge even less. If you're really lucky, the person you wind up with will be really great at it and that price will be a steal. (In which case they won't be charging that little for long!) If you're less lucky, they'll really stink at it, and after a long, drawn-out series of back-and-forth negotiations and remixes, you'll be out $300, looking for someone more capable to start over from scratch.
Of course, it is possible to bring down the per-song cost of a mix if you're getting several songs mixed as a batch. Alternately, if you're not in a rush and willing to have a hobbyist or semi-pro mix on their home pro-tools rig nights and weekends, your could also save some money that way and get some ok results if you happen to hook up with a someone who's really talented.
Anyway you slice it, that old adage from the construction world applies here: "Cheap, fast, good. Pick any two."