The crystal is already included. Of course you need an antenna and several other parts and most importantly communication between base and rover for correction data. Integration is the elephant in the room here. Ready to use boards go for like US$ ~ 300 (without board to board communication).
Admittedly I misread your post - you asked for $ 100 a pair. But a few hundreds is possible - at least in costs.
Admittedly I misread your post - you asked for $ 100 a pair. But a few hundreds is possible - at least in costs.