For a small busy developer trying to make a few boards, if they can't push a button and walk away its not saving them money or time.
Honestly that's a pile of BS. When was the last time you hand placed a board? ANYTHING is better than hand placing boards. For a typical 10x10cm board with a few hundred parts it's at least an hour of continuous, eye straining labor. Why do people buy manual PNP machines? Because it saves the physical strain of handling hundreds of tiny parts. If a PNP machine can automate 99% of the process then it's a win.
One thing I can tell you is the factory rate to put small batches though our line is less than a developer.
That does not say much. No company employs "developers" to do manual assembly work. The real killer of in house assembly for prototyping is automated PCBA services like JLC SMT, which is only around $10 setup costs per board for assembly.
SMTech, funnily enough stands for "surface mount technician" which is my job title, so I hand build boards or partially hand build boards whenever I can't use the machine to do it or when its not worth doing. Yes its not fun, and I go to great lengths to avoid it and not an inconsiderable amount of complaining if if is made necessary when it shouldn't be. However even a good machine takes time to setup for a new job, and for low volume your parts might not come in packaging that suits, nice tho it is to be able to define all your little strips as trays, its a slow and time consuming process so often its quicker to whip out the tweezers or use the manual P&P.
There is a fine balance in there between the complexity of your boards, the number you are building, the pitch/package of the devices you are using that move the balance around between the best options for a given user, machines that are fiddly to setup and use will shift that balance far more quickly than you might have planned for when you bought it. Equally if your machines placement accuracy is such that things need "a little tweaking" afterwards you again lose the time you think you were saving. If your boards almost always are 100s of parts and small enough to fit on these machine and you need it often enough, sure it could be worth it, however given some of their quirks and very low feeder count, I would suggest loading the machine permanently with all your very common stuff, and putting a manual P&P next to it for everything else (tweezers suck). However if you build simpler stuff or very infrequently there's a very good chance it it really isn't saving you anything, and you should just be honest with yourself that you want a new toy.
As to the chinese assembly services, I think if we ignore the semi secret chinese market only ones (like the JLC you refer to) they are charging much more for the setup of an assembly job (not that they itemise it) than $10. If I put in some made up numbers into Elecrow for instance I get a number really quite similar to the one we would quote ourselves. It even scales in a similar way with volume.