Everyone has a prefernce. Depends on your level & what your willing to do. Some producers do send to engineers to mix (like myself) because they want their sound to be crisp and to-par with these other mixes. You may find an engineer who is willing to do it for free, but keep in mind that just because its free won't mean its good. Their "free mix" may sound just the same as your own mix so keep that in mind.
Another thing is the engineer won't have a clear idea of what you want. So they may just make the kicks knock and turn down the synth that you wanted as the main element. But as everyone said it just takes time and lots of practice. I see a lot in this forum people asking how Johnny Juliano or Superstar O get their mixes so clean/punchy. It doesnt take a lot, they just make space for their kicks/808s and hpf everything else, start with quality samples, and the then brickwall everything when they "master."
At the end of the day if your on Grand Hustles label & you make a beat for T.I. and he loves it, your still going to have to give the tracked out version to an engineer for proper mixing. At a smaller level just keep practicing or hire someone to do it. I'm sure myself or any of these other great engineers can help you along your way. Peace.
Mixing Engineer & Record Producer ~ Twitter @Trilltrax