You cannot "retrofit" a JD cab. The tractor is basically built around it-that's why they are so nice!
An aftermarket cab will be anywhere between $2k and $4k depending on options, BUT it will be louder, the heat won't work as good, there will be no AC, and it will not look factory.
If you want a cabbed machine-then pony up the money for a factory model and you won't be disappointed.
Go to JD owner/operator form On the third page was a post " I bought a cheap enclosure... " I believe it said in there that they had them for other model JD. V