Hire someone who's work you like.
A few quick comments:
- a basic mix of a low track count song might only take a few hours; most people allow a day, sometimes a day plus revisions. Especially if there's any tidying up needing doing to make it as amazing as possible.
- don't hire "a studio" - hire an engineer.
- most people working on a song rate don't necessarily want people with them - that's the point, you work when it's convenient to you, and having someone with you will slow you down, so most people would want to go hourly at that point.
- US$400 is on the ok-spectrum - I wouldn't call it "above average".
- if someone says they're "grammy winning" they're probably not

you can find the name of anyone who's actually been "grammy nominated" or won on grammys.com - most people aren't there. A lot of the time at best they had a bit part on a record that won a grammy somewhere...it doesn't mean much (and that's speaking as someone who genuinely IS grammy nominated! I recorded a vocal on a nominated record..my "engineering" is not the thing that got them nominated!). So don't read too much into that - no-one who actually won the engineering grammy is working for $400/song on Soundbetter.
- that said - if you can find someone on there who's got proper credits on songs you like - why not go with them?
- there's "official" resources for credits like Jaxsta.com - which only has verified credits. So try looking up a song you like, see who mixed it, see if they're available. I'd go that direction above just going to a local studio.
Does that help or confuse further?