The original poster asked, "How much does a top pro mix engineer charge...?" I can't tell if you're trying to be funny or what, but there is literally not a single top-of-the-line pro mixer charging anything near $300/song. If we're talking the very top pros, they are in the thousands, period. And like @paularthur said, you might be able to book one of their assistants for $500/song. One of my friends specifically instructs his assistants not to charge less than that.$300 is on the high end for the mix of a single song...you don't know what you're talking about, son.
Not impressed by Ghenea either. So suck it.