I am running a JD 450 monitor on the 12 row and a JD 350 on the six row. I did use the radar readout form the tractor now I just use a GPS speed sensor. It was way cheaper than a new radar when my one quit. You can get them from AG Express.
You need to call Roger Kalsem at Waterloo Iowa. He deals in used corn planters and implements. He should have a used monitor that would work for you for way less than what you are talking about. Your harness and seed tubes should work on the JD 250 or 350 monitor.
Give Roger a call. If he does not have anything email me I can check at my old dealership ans see what they may have left.