I'm very firmly in this camp, being deeply reluctant to spend money on advertising. That said, of the two books I've so far self-published (following on from a primarily trad pub career), I think maybe a sixth to a seventh of my overall sales can be tracked to ads, which in my case are just about exclusively AMS in the UK and US. In raw numbers, that's an overall spend since March 2018 of £320 for sales of just over £700. And, of course, that sales figure is for the total price, rather than my slice of the income, so while I've probably made a profit, it's not really much of one for the amount of effort involved.
(I should add here that the above figure is purely Amazon UK. I've barely sold more than a handful of books in the US via advertising in that same time period, and when I say a handful, I mean a handful.)
I've tried spending more in the past - pennies, compared to some of the numbers being tossed around here - but every time I did it didn't prove worth it. I got no extra sales, just more expensive no-sale clicks. I've got decent covers, and spending more on the covers with each new release to get the best quality art I can afford.
So, yeah, reluctant. Mind you, every now and then I read about some guru or other who's handling a whole range of AMS accounts for authors and helping them make money. There are paid advertising promotion services that can supposedly be useful, and help improve sales significantly, but I can see people here and elsewhere are generally leery of them, either because their effectiveness is questionable or because they're insanely expensive.
One in particular, Books Go Social, I've thought of signing up for, but something keeps pulling me back at the last moment (I'm interested in their Amazon ads services and Netgalley access, much less interested in their Twitter accounts and email lists, which they push quite heavily but which, according to some anecdotal evidence I've gathered, are no use). But they're not too expensive, so I may yet give them a go.
Mind you, the fact that advertising accounts for just a sixth or seventh of my overall sales suggests that organic sales on Amazon are a gold-lined pipeline all in themselves. You can perhaps raise visibility using ads, but perhaps, in the end, it really comes down to compelling writing, a cool idea and a good cover.
I keep thinking of a friend who's got a dual trad and self-publishing career (he works for a games company). A while back he put the first in a new series out and due to various life events was unable to give it any support or promote it in any way whatsoever. When he was finally able to check how it was doing, some time after its release, it was in the top five thousand or so on Amazon.com with zero promotion and raking in stupid amounts of money.
The only real rule is that there are no rules.