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I reckon you've hit it. I find grammar comes out in the proof-reading. And, doing my maths, in Australia it's about the same price. $5 to $10 per 1 1/2-spaced page! (I think that's $3.50-$7 US.) That's about 400 words so that's over 2cents a word, or 1.6centsUS.
I was employed for a few years as a line editor/proof reader for a small press publisher. If a work needed developmental editing, then the publisher would not accept it.
That's what I use alpha readers for, really.