Every major chinese PCB supplier has offered very affordable PCB assembly ("PCBA") for several years now where you give the gerbers for the boards along with a BOM with part numbers from a mutual supplier catalog and p'np / solderpaste mask data and they will load up a p'np machine and oven run a small batch of boards for you.

e.g.:

https://jlcpcb.com/smt-assembly

https://www.pcbway.com/pcb-assembly.html

That price is including PCBA. Without it would be <$1/board