The average retail on a D-15 is about $3500. High trade-in is about $2000.
The 210 and the 220 were sold simultaneously so they may have both replaced the D21 - don't know. The average retail for a 220 is about $7000 and High trade in is about $4000. Don't know the cab value.
I don't believe any of these are rare. But then I've seen guys call an A John Deere rare. They made about 25,000 D15 tractors and less than 2500 each of 210 and 220. I have a Massey Ferguson that they only made 500. When are they rare? I don't know. But, I'd think a rare tractor would have less than 50 copies left.