I don't have input for your original question re. low cost RTK, but I'm wondering how you visualize the combine RTK/autopilot working? Have you used autopilot already on corn combining?

The only thing we found that worked well for corn combining (8r) was the TrueSight guidance. It has no connection to RTK or any other type of radio/gps guidance. Purely mechanical. The RTK autopilot would help find the correct rows..but that's all. That's our experience anyway. The TrueSight works as purely hands-off driving.