Editing profession as a freelancer?

A prospective client is interested in having me edit his short stories for grammar, spelling, and sentence structure. I'm interested in knowing the typical going rate for editing and proofreading services of this nature. I'm new at this, just doing it on the side to earn extra income. I have a strong writing background and I'm qualified for this work but I want to be accurate with the fee that I present to him when we meet tomorrow. I wonder if $5-10 per page is too low, too high, average? I live in NY. Do editors charge by the page? Please give me some insight. Thanks.