Yes, if your conversion rate doesn't make sense for you, don't do it.
I'm not going to go arguing how to make $500 into 10 sales. It depends on what you are selling, what the competition is bidding for ads, your lifetime value of a customer, etc.
But you asked how you can go to $500 -> 10 sales?
2500/30 = ~83.3 clicks/day
$.20 * 2500 = $500 :: 2500 clicks
2500 * .004 = 10 :: 10 sales @ .4% conversion rate
This is really not a strange/weird/unusual scenario for a combination of 20+ low competition CPC ads on Bing, Google, etc. Given small budgets, that is what you'd aim for as you don't have the money to compete with the high cost keyword bidders.
I'm confused why you think that is hard to achieve?
These numbers aren't particularly optimistic given most people aim/expect a ~1% conversion rate, so if it was .8%, you could double your CPC, etc.
You keep throwing theoreticals around, inventing numbers to match your narrative. "If horses were 10 feet tall, then we'd be able to ford this river." A 10 foot tall horse is called an elephant, and we can't afford elephants. How do we ford this river with the one horse we have? And your answer is, "well, if animals were 5 times larger than they usually are, then I don't understand how you don't understand."
I'm saying it doesn't scale linearly.
And yes, I've done this with $500/month as the spend. If you go through my comment history you'll notice I used to work entirely with small businesses as a contractor which I've mentioned at least a couple times I believe.
I wouldn't use up-to-date hypotheticals from my current job given its a scale almost no one on HN would operate at. :/
I'm not really sure what your conviction this isn't possible comes from?
Obviously, $50/month/sale is not going to work out for a book that sells for $10 but that is a different problem.
If you were starting a project on your own, tomorrow, and the only money you had was the money in your own savings, what would you do? You said $500 gets you 10 sales. Where? How?