done
Fyi, "What's the single most important thing fo (should be for) a mastering engineer...."
My answer to that question btw is "Ensuring solid overall volume without killing my dynamics."
And for $30... if the Master is good, then there's nothing wrong with that price.
The reason I wouldn't pay anyone to master my material is because, "I use Ozone 3 and do my own Limits and they sound fine on everything I reference my songs on."
I originally had a couple tracks mastered back in '05... then got interested in figuring out what needed to be done for that process and after learning that I really don't like someone EQing my work, and doing Multi-Band Comp on my hard earned mixed down track, I decided it was time to learn how to do my own Masters.
After learning that doing a simple Limit on a splendid mixdown is all that's needed to satisfy my ears, I decided I'd do it myself for life.
If you can do it for $30 and make a profit for your time, then I don't see why anyone would have an issue with that. If you're having to try to make a poor mix sound good, then $30 wouldn't suffice for your time imo (I wouldn't even mess with that kind of work but that's me), but if someone is sending you a fantastic mixdown and all you have to do is roll off 30hz and below and Limit, then yeah, $30 sounds like grand theft money.
g/l with your business