When you get that email, particularly if it's your first book, make sure you are well-rested and in a positive frame of mind before you open it. If it’s your first book, the editing process will be hard on your ego, but remember, the editor’s job is to make your book better and help you learn the craft. You are paying for them to give you critical feedback, not to pat you on the back and say ‘good job.'
The job board is NOT a big reason to join the Den, especially if you’re a new writer. It’s a higher-end board that doesn’t post any low-priced gigs, and is a good resource for mid-career writers. But in general, the board is a real sidelight to the point of joining the Den, which is to access 300 hours of training+, 24/7 forums for support, live events where you can ask questions of experts, and more.
If an editor’s rates seem skeptically low, send an editing test, ask for references or a sample, place a hidden message in your post, and see if they understand the different types of editing. If an editor passes all of these tests, even if she’s a newbie, give her a try. This could save you hundreds of dollars and help you find a skilled editor who is competent and affordable.
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.