$50 radio 5km out to sea?!!

Yes, but not really practical past 4km.
Gear used:
FS-i6X radio (Banggood): https://goo.gl/LB1vs6
Telemetry radios (Banggood): https://goo.gl/Y1PwKS
VTX antenna (Banggood): https://goo.gl/uas4WC
VTX (Banggood): https://goo.gl/WE1Dc6
GPS module (Banggood): https://goo.gl/V4HAN1
Helical antenna (Banggood): https://goo.gl/QMkTWz (ymmv...)
FPV monitor (Banggood): https://goo.gl/Kv4VQ5
LiPo battery (HobbyKing): https://goo.gl/9WwdKK
Mini-talon plane (Banggood): https://goo.gl/LHtd5r

The 900MHz antenna on the ground station is one that I got with the u-blox RTK development set, I don't know where you can buy them.
The 900MHz antenna on the plane is this, but seems to be discontinued (HobbyKing): https://goo.gl/HB7jK5

Видео $50 radio 5km out to sea?!! канала iforce2d