It's hard to say, but if you were to put money into buying a Cooper S box in rebuilt condition you're looking at about $300-$800 depending on condition and degree of reconditioning required.
I would pay anywhere from $50 bare casing to $200 for the box in unknown condition. There's not much point in paying big bucks for a box that needs new gears or selector forks which will end up costing you BIG BUCKS. Basicaly the case is a 12G1128 which are the same cases as mini K boxes. The only difference being 2 gears which you can buy fron UK for about $300. So that is the monetary difference.
So to a degree rarity of G/B's are not as high as 9F blocks. ( I have 2 12G1128 cases and have seen many around but only ever eyed 1 or 2 9F blocks)
Engine itself is a whole nother story as they are getting increasingly rare. Really they are no difference from any other 1275 block cept for originality stakes, someone with a $20,000 Cooper S will pay $X000 for a matching engine get my drift?
The other thing is the crank, EN40B cranks are SUPER rare. Hope that helps. Think of it as an investment, they are getting increasingly rarer. in 2020 there will be hardly any left!
