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I'm on a discussion for indie authors on Linked In, and one of the points brought up is the cost of proofreading.  Several of the authors chose to ask friends to proofread their books, only to find out upon publication that not all of the mistakes had been caught.  Another author sent a copy of his book for review.  I noted in my review that there were three or four typos per chapter.  Again, he was chagrinned because he thought he had caught them all.
If you are looking for a really good proofreader, but you can't afford to pay a professional, another alternative is to contact your local service center for autism.  People who have Asperger's Syndrome, a high functioning form of autism, make the best proofreaders.  I know, because my brother has Asperger's.  He's in the top one percent of IQ scores, but his social skills are retarded, and he tends to be a loner.
In a recent article published for parents of autistic children, a psychologist noted that proofreading is one of the best jobs for someone with Asperger's.  These people think like a computer; on/off, up/down, right/wrong.  That's why most people find them to be frustrating, and why they can program computers without a single swear word coming from their lips.
One of the characters on the TV show The Big Bang Theory has Asperger's.  He's very smart, but frustrating because he is totally logical, and doesn't respect peoples' feelings.
But, if you want proofreading done to the letter of the law, this is just the sort of person you need.  Forget about asking your friends, your child's English teacher--if you want it done right, ask a nerd.