I have two TERSUS GNSS GPS receivers for sale (BX306 and BX316), I originally bought them for testing on a traditional heli, both are like new and sell for around $1,850 new, the antenna is $500 new. I also have an AX3703 antenna which is ideal for mounting on the aircraft. I can sell these items either separately or together.
BX306 $300 https://www.ebay.com/itm/312616097789
BX316 $300 https://www.ebay.com/itm/312616154094
AX3703 Antenna $60 https://www.ebay.com/itm/312616948398
I also have a hot shoe adapter and external data logger that I throw in with purchase of either of the GPS receivers if needed at no additional cost.