If you have a good command of the English language, and your grammar isn't atrocious, you don't need to shell out hundreds of dollars for editing. And proofreading you can either do by yourself by reading your work aloud, or have someone you know read it over for you.
Two Points:
One, this assumes that people are good judges of their own abilities. I've found this is not the case. I can assure you, as an editor that reads HUNDREDS of submissions a month, far too many authors do not have a good grasp of the basic mechanics.
If someone judges themselves as having good command of English and decent grammar, who's to say they're right or wrong? Based on whose criteria? The market will tell you if something you're doing isn't working.
But, there is no data to support the notion that one's 'good grasp' or lack thereof results in greater/fewer book sales and/or a higher/lower overall ROI. The reason why is because most if not all of the above is unknownable and unquantifiable.
And if that's the case then how can anyone state with any certainty that authors must pay a professional editor for the sake of viability and/or validity within the publishing market place?
Sure, you could collectively point to all the millions of sales and high ROI enjoyed by those who have paid for editing, but as I've already stated, how many - if any - of those sales, and what percentage - if any - of that ROI can you quantifiably attribute to the said payment(s) for said editing?
What would the opposite figures be? How many more/fewer books would an author sell with/without paid editing? How much greater/lesser would an author's overall ROI be with/without paid editing? That's just it. It's impossible to predict.
How often do consumers in the market place know how much, if at all, a given work has been professionally edited versus self-edited? How much or to what degree is this knowable?
And, if it's reasonably unknowable just as I assert, then how can anyone surmise in this context that it's rational to tell anyone that they "must" do "x" in order to achieve "y"?
Having a command of the English language, so to speak, for purposes of speaking is not the same as having a good grasp of mechanics for purposes of writing.
You may be objectively right (who's to say and based on what specific criteria?), but where is the data to suggest one's more frequent use of "speaking" type English (however that is defined) in their books results in fewer sales/lower overall ROI than those who more frequently use "writing" type English (however that is defined) in their books? This is another unquantifiable aspect and therefore, in my opinion, not an area where one could rationally or with any certainty tell someone else they ought to be spending money on professional editing.
Two, this assumes people aren't blind to their own work. I AM an editor, but I won't proof my own stuff. Because I know I will read what I MEANT and not what I actually WROTE. Now I can proof my own stuff if I sit on it for a couple of months and come back to it with fresh eyes, because that distance allows me to read my work as if I was reading someone else's stuff. But for the OMG I HAZ TO PUBLISH NOW crowd that wants to hit the publish button as soon as they are done? Not gonna work.
The market will let them know if it's working. If the market tells an author that something isn't working, and that author decides maybe a lack of professional editing is the issue, and that author came to me for advice, I would tell them by all means pay an editor if you think that's what's lacking or hurting your sales and your bottom line - but do not feel obligated, as though you "have to". You don't because I would say in this squishy context there is no "have to" because the objectivity needed to determine such a thing, in this area, does not exist. End of the day, it's your prerogative, your money, your career.
My judgment of their writing might be right, might be wrong, just as their own judgment could be either - just as a professional editors judgment could also be objectively right or wrong - but, that's the point. Who's to say when it comes to what works best for that author's budget, or what the end result would be in terms of sales and ROI?
This is why I bristle, especially, is at the notion that this paying an editor simply "must" be done by any/all/most. There's no objective calculation that can be performed to support this. We're talking about something amorphous and unknowable.
I think many authors enjoy healthy sales numbers and a high overall ROI without paying an editor, regardless of whatever the objective truth might be about their writing ability and/or their own ability to judge their own writing ability. The market's feedback is important here even if it isn't strictly definitive.
Of course, the opposite is also true. There are many authors who sell few books and/or have a low overall ROI strictly because they're not paying an editor and perhaps because their writing ability is a problem and/or their inability to judge their own abilities or lack thereof is hurting them. But, even if in a vacuum we could tell this was objectively the case, there's no way to definitively quantify that it is.
Yes, it is happening, but there's no way to determine to whom and to what degree that could lead anyone to concrete, actionable advice. The market's telling them this, sure, in as much as it can, but it's not doing so explicitly because of its unquantifiability - that is, the lack of direct correlation anyone can objectively elucidate between low sales/poor ROI and a lack of professional editing is a thing. That's what makes it all part of the author's amorphous trial and error of finding what works and what doesn't. It's a constant struggle everyone engages in to navigate this business's fog of war.
Which means that either of the scenarios above can be the reality or a given author's reality might be somewhere in the middle. But, again, it's that all/most of the above is unknowable or unmeasurable that leads me to why I do not think it correct to insist or automatically presume that any/all/most simply must pay for professional editing.
To me, it's too squishy and amorphous a thing to speak on with any level of certainty, and certainly not an area to which I would say one ought to automatically commit dollars.
I'm fairly conservative when it comes to money so, I'm leery about spending in areas where quantifiability is a problem. Geez, did I pick the wrong industry or what?
But, it explains why I'm reticent to sign on to advice that says anyone else ought to commit money to something that I see as being part and parcel with the fog. To me, I'm not certain that's sound.
That said, if you have plenty of disposable income, and you think you need professional editing - go for it - your judgment is your judgment. If you think that it will eliminate one checkbox on your list of trial and error items, go with God. However, I don't think anyone ought to feel obligated to pay for editing from the outset, and I think many authors can succeed without the paid version of it, but in the end it's up to you and you alone regardless what anyone else says.