Trimble GPS receivers go for around $10k (USD) [0], so I'm not surprised farmers try to build their own devices. Here's one who built an RTK GPS system out of a Raspberry Pi (and various other parts) for £250: https://www.youtube.com/watch?v=Kwxpo04AC5Q

0 - https://trimble-exchange.com/store/item/used-trimble-r10-gns...