What's the formula though for determining what to charge people for "monthly" maintenance please? Online info is showing from $35 to 5k.
Admins - we may need to split this thread.
The million-dollar question

My mechanic charges me $65 for an oil change. There are plenty of "national franchise" places that'll do it for $19.95, and even one guy in town that does it for $9.95. But I spend $65 for what, in theory, is the same service. But it's not. They check and fill other fluids, check tires, give a once over in the car, clean the windows, vacuum the driver/passenger floor, and give a quick wipe down of the dash with ArmorAll. But it's advertised as the same service - "oil change".
I can tell you our price - $150/month - but again, we're not doing just a basic push of a button. Very few plugin update companies make a full copy of the site, test it on a separate environment, and verify everything works, and then proceed on the LIVE site (during off-hours), and re-tests again. Most companies are doing that on the fly in the middle of the day, and when they run into issues, then they figure out a solution. We also have a list of all plugins and versions installed on client sites, then cross-reference that against vulnerabilities announced, and update as patches are needed. Not every plugin version released warrants an actual site update, so we bundle them together when needed.
Can I get all that done in an hour? Usually yes, but it depends on the site and how large their site is. If they have 4GB of images, copying that site to a staging site and running updates would take longer than a normal 5 min copy. If they're a large eCommerce site, more time is often involved, and pricing is adjusted.
The important question -
do I enjoy it? No. There's nothing exciting about it for me. It's for that reason that we only offer it for websites that we have built or SEO clients that we are actively working with. It's a nice upsell, and it allows us to make sure that plugins are at the latest release version, but that's it.
If a client chooses not to do updates monthly, and they want to do it on their own, have at it. If they want to contact us every few months and get it done, no problem, we track hours and do the work. It'll cost them less, in the long run, to do it monthly but it's up to them.
Do we compete with hosting companies offering plugin management? Kinda, but not really. They do everything automated and it's up to the client to complain when something is wrong. There's no proactive management or testing done usually.
All other updates on the site are via prepaid retainers (not monthly retainers, just a block of hours). We track everything. If it's a task we're doing for a client, it's tracked and comes out of prepaid hours. If it involves thought (redesigns, structure, content writing, enhancements, or troubleshooting), we try to provide estimates on timelines. Some are unpredictable, like a site hack - we get hours on the books, roll up our sleeves and get to work.
So the price, much like with anything else, is really determined by you on what you need to be paid to make it worth your time, and then evaluating what the market can support. When I ran a hosting company, I used to sell hosting at $4.95-$44.95/month. We had hundreds of servers and thousands of clients. We truly were a white-glove service as our staff offered PHP troubleshooting and assistance to clients. I don't know of hosting companies that offer that today, and certainly not for under $45/month. If I was to launch a hosting company today, I would put pricing between $75-$200/month. Yes, there would be fewer clients, but they would be a different kind of client too.
Can you offer maintenance plans at $45/month? It depends on what your hourly wage needs to be, and cover software overheads etc. If you got it all done within 1 hour, would you be happy with $45/hr. If it took 2 hours, are you happy with $22/hr? Can you offer more routine maintenance like content and picture updates on the website, maybe trickle into GMB Posts or Facebook Posts as additional services to boost the monthly maintenance price?
At the end of the day, the price is all about what you want to make, what you're offering, and what the market supports. My mechanic is not always busy, but then again, he only needs 1 customer, whereas the competition needs at least 4 to make the same money after materials.