PCB manufacture the traditional way is very labor intensive. I have assumed that places like JLCPCB are very highly automated. Very little engineering seems to be performed before production. It's my impression that in the US when you request a quote a human inspects the Gerbers to estimate a price. When the Gerbers are prepared for production even more human labor is required. This is when US fab houses will come back with questions, either to clarify my intent or to highlight what they believe to be problems.
OTOH, JLCPCB creates quotes by computer. And, though I have zero first hand knowledge, my impression is that there is a minimum of human involvement in preparation for fabrication. In my mind I imagine relevant layers being identified automatically. Superfluous features outside the PCB area (title blocks, etc) are stripped away automatically without ever being read at by a human. Panelizing is done automatically. Possibly a technician inspects the result quickly before the job is queued up on one production line or another. The biggest labor is in final inspection, cleanup, and packaging.
Even if my mental image of what goes on in those factories is completely wrong, that sort of thing could be done in the US and elsewhere, if it hasn't already. That's the only way prices could come down significantly. The downside is that your Gerbers don't get inspected very well, but I could live with that in exchange for a drastic reduction in cost and turn around time. (Actually, besides price, one of the things that has turned me off of domestic sourcing of prototypes is long turn around time, much of which is consumed by inspection and questions. I kinda wish they'd just fab the damn things and send them back ASAP.)
Having gotten all of that off of my chest, what North American fab houses can be recommended for prototypes?