For a JD system you can get a brownbox (with keycard) for around ~$2,900, a iTC Receiver for ~$900, and if you talk to your dealer you should be able to get an ATU 200 for around $900. Plus a couple harnesses and mounts.

So you'd be at around $5500. Don't know if you could do it cheaper with Deere unless you found these items cheap on an auction.

I cannot speak for other systems, I'm sure there are other ones that offer cheaper solutions.