First I would like to say that even if you are the best grammarian you know, having a professional look over your work is always best. With that being said, before you drop $$$ on that professional look you should at least do some editing on your own.
If you are going the traditionally published
That’s why in my writing process, plot holes is the first round of editing. Unless you outlined you story to death beforehand, it’s very common to find glaring holes in your plot or actions that don’t really make sense to your characters personality or motivation.
This first look is strictly about tightening up your story. Don’t worry about grammar and punctuation just yet. Why not? I personally think that it’s better to stay in the free form mode when editing for the plot. I am more creative that way and my new writing may not be so grammatically correct. I rather catch all of that at once, if I can.
Bonus doing some self-editing can save you money on professional editing. The more polished the draft, the less time and corrections for the editor to make. Depending on the length of your work and how your chosen editor charges their fee, that could mean the difference of a couple hundred to a couple thousand dollars.