And then come the WP plugin license fees, which easily quadrupel that cost, if not more. Just the other day I have seen a WP instance, that has >50 plugins installed, some of which cost 800 Euro per year. They are lock-in traps for hapless marketing and other non-technical people, who are convinced that they definitely need this super-duper plugin.
And then come the legal fees for making the site actually conforming with the law, such as GDPR. Those fees are increased, because of people wanting to do stuff they need to declare to visitors of the site, for which they want reassurance, that all is well.
And then come the costs for paying a dev anyway, to fix things that they break or that become broken over time.
So no, 9.99$/month are very very far from a realistic price these businesses pay.
Well we have hosted 10 small business WP sites per $10 DigitalOcean droplet for the last decade. There are not additional plugin costs on any of them. And there has been no real maintenance needed.
I'm not saying WP is great. Taking over a WP project from someone else can be daunting in tech debt and weird choices. But in terms of having a simple brochure website for businesses that get < 10k weekly visitors, it's pretty quick, cheap, and easy.
Reality check: So you have no forms any user needs to input anything into, because if you had any, then you would need a spam protection, lest your customer gets tons of spam. Cheapest one is probably Askimet, but that costs money for a business. So either you are talking about little blogs, or not businesses. In any case, probably real small sites of purely informational character without user input, otherwise it would cost something. Or you are letting your customers get spammed.
No real maintenance? So either you let your PHP version and plugins become outdated, or you sooner or later have to fix things breaking. Maybe you simply did not notice any breakage, because you don't do maintenance for customers?
A brochure website? Does that mean people enter their e-mail to be sent a brochure? (Then paragraph 1 applies again) Or brochure meaning, that you merely display information on pages and that's it?
I think for small info sites what you describe can be true, but for anything slightly larger not, especially not for small businesses.
And then come the legal fees for making the site actually conforming with the law, such as GDPR. Those fees are increased, because of people wanting to do stuff they need to declare to visitors of the site, for which they want reassurance, that all is well.
And then come the costs for paying a dev anyway, to fix things that they break or that become broken over time.
So no, 9.99$/month are very very far from a realistic price these businesses pay.