Please, unless you are skilled in the finer details of writing and can polish up the little nit picky things yourself, like commas and basic punctuation, please...PLEASE do not pay for an editor to go through your manuscript. It will be a waste of your money and the editors time if your grammar is so bad they can't focus on your content.
Proofreading is vitally important. Learn to do it yourself (or have a friend help you) and save your future editor some headaches and eyestrain. You don't have to be an English professor or anything, but you should know the basics. How to structure a sentence, a paragraph, and a chapter. You should know how to use commas and full stops. You should also know the difference between a comma an an apostrophe. If you want to get the best bang for your buck, you will be able to clean these issues up yourself before you send your baby off to the
It doesn't have to perfect. If you could make it perfect by yourself, you wouldn't need to employ the services of an editor. But you should work hard to make it the best you can before you pay someone to edit it for you.
Some people are also under the impression that an editor will fix your manuscript for you. No. They do not. You send away your manuscript and what you get back is your manuscript. They suggest changes based upon the rules of grammar, they point out spelling issues and structural issues. Depending on the depth of the edit, they may even make suggestions about content and story arc. But they do not fix these issues for you. What you get back is a more colorful version of your manuscript. Things will be highlighted. Things will be crossed out, there will be notes in the margins. It won't look pretty. But it is SO worth it to have a good editor look at your work...WHEN it is ready.
A good edit can make all the difference, so make sure you have it edit ready before you shell out your hard earned money.