John,
Take a look at this Base/Rover kit for $1,999 :
You can also read my RTK discussion here:
https://community.pix4d.com/t/4703-Discussion-on-RTK-GPS-Systems-#gsc.tab=0
In my experience, the field work required to achieve 1-2 cm Model accuracy requires occupying each GCP at least twice with RTK GPS (best if at different times of the day), capture enough low altitude images for a ground sample distance near 8 mm, and use plenty of MTP’s.
You can find a used Trimble or Topcon Professional unit under your $6,000 budget.