there are alot more to the cab than just the cab unit itself. The entire area from firewall foreword is different.
I actually thought about adding a Curtis cab to my old 870, but the idea of no AC discouraged me. I wanted to leave it on in summer due to bees and hornet issues here. And id roast without the ac.
When i purchased my Kioti, the cab ran $7,000 over same model without. I didnt even consider getting one without.
$7,000 seems like alot for just a cab, but that also included 2 sets of wipers, all that glass, a/c and heater, lights, sound deadening equipment, stereo , speakers,antenna, mirrors, larger alternator, ac compressor and hoses,heater hoses, larger radiator, plus stuff i dont even know about. Heck i doubt i could build one (if i had the skill) for less than 7 grand.