nRF52s are definitely not extremely cheap, they're high-quality fully-documented dev-friendly chips with lots of onboard resources. If you want cheapest BLE, there's some utter crap from Dialog Semiconductor that comes with a proprietary SDK in ROM and one-time programmable (!!) memory for the application (external flash can be used in development).

And you DO NOT have to use the Nordic SDK, there are lots of better and fully FOSS options — Apache Mynewt, libopencm3, Rust nrf-rs…