I agree, it still is a rare car compared to the number of automatics that they sold.
It's main competition in 1965 would be the Impala SS and if you want to compare the number of SS four speeds built vs Sport Fury 4 speeds, you would say quite rare for the Fury.
If you flip it the other way and compare the Plymouth to '67 300 4 speeds, then it's a bit more common.
Bottom line, it would be cool to have, but the 4 speed doesn't add a huge price premium to the value of the car. If it's a nice, clean car, the 4 speed adds maybe 2 to 3 grand to the price. Sometimes you will see people asking $25K for an average car just because it's a 4 speed, and that's too much.
Jeff