Professional Editing, is it necessary?

My oldest sister messaged me the other day on Facebook. She asked how my book was coming. She does this from time to time at random. It just so happened, that the message got to me right after reading another rejection letter. Needless to say I wasn't in the best of mind-frames at the time to answer.

"16, rejections now... It's going...," was my reply.

Mental note- Allow at least an hour to reply to anything after reading a rejection letter.

She responded back quickly with, "(insert other sister's name here) sent hers off to a book therapist. You should talk to her about it."
(Yes, thats right there are two aspiring authors in the family!)

Thankfully I waited a little before replying again. I get cranky when I get a rejection.

I've thought of book doctors, professional editors, or whatever you want to call them.

I've wondered if it was really worth the investment to send out my work and have it professionally edited?

Let's face it, it's not cheap!
I've seen quotes of $700-$1k for an 80k word manuscript. That's a lot to invest in a gamble.

What are the benefits and are they really worth it? Will an agent take my work more seriously? Will it really give me an edge?

Professional editing is no guarantee. If the story doesn't catch the eye of an agent or editor, the money is lost.

Still, I really want to be published. I've submitted my work to critiquing groups, I've bought books on grammar and punctuation (let's face it, we all know I fail on this subject), and I've tried to go over my work with a fine tooth comb.

Does it really make the difference to professionally edit? Has anyone out there in the void used these types of services? Any recommendations?