How much does a MB of L-n cache cost?

I don’t have the answer, but you would want to measure dollars to buy it, and nanoseconds to refill it.

That's true, if you're buying OEM ram for Dell or HP servers, it's more like $10-20/GB. However you can buy Crucial ECC DDR4 ram for $6/GB, so there's a hefty OEM markup.

$10/mo is far less than the cost of thinking about the issue at all.

Yes, but. Suppose you build a thread-per-client service before you realized how much you'd have to scale it. Now you can throw more money at hardware, or... much more money at a rewrite. Writing a CPS version to begin with would have been prohibitive (unless you -or your programmers- are very good at that), but writing an async/await version to begin with would not have been much more expensive than a thread-per-client one, if at all -- that's because async/await is intended to look and feel like thread-per-client while not being that.

One lesson I've learned is: a) make a library from the get-go, b) make it async/evented from the get-go. This will save you a lot of trouble down the line.