I'm an editor by day, and I'm the first one to say that the best thing you could do is do your own editing. If you're not positive your story is the strongest it could be, it's probably not.
I agree with Dave: joining a critique group is probably going to be your best bet. But most likely, readers will give you overall feedback (does the story work overall, etc) as opposed to copy edits (grammar, punctuation, consistency, style). If you need to brush up on grammar, and CHICAGO 15 is too daunting (this is the bible of style in the publishing industry), I recommend the more user-friendly GREGG REFERENCE MANUAL.
As an example of expenses, when I did freelance editing, I charged $2/page for copy edits, $3/page for developmental edits, and $4/page for both, for 12-point Courier New, double-space. (And I stuck on an additional $30 fee if people didn't submit their work properly formatted.) I'm not sure what rates are anymore. (I also don't do freelance editing for individuals anymore; one of my clients simply didn't pay me.) Note: you may find that some freelancers will insist on 50% upfront.
Anyway, good luck with this.