You know, since these machines are not equipped with Hvac, this might be the silliest idea I've seen. Kudos to you if you get buyers.
Meanwhile, my damned MSC half doors for my 2022 1000XL have exceeded the 4-6 week pre-order timeline (now projected 2 more weeks lmao) and I am ashamed to say with cost, shipping and taxes it ran me $1,600 
I think you may be missing the point of cab designs like this. Many tens of thousands of this style of cab enclosures have been installed by UTV owners for decades worldwide. Sold not by just us, but by multiple manufacturers. Our cabs are fully modular, meaning each panel connects to the other & which lets you warm or cool your cab when you want. No need for Air con on a hot day if you open the vents in our windshields, or remove the windshield panels, zip back the doors & zip open the back panel. The doors, back and top can be removed & re-installed quickly & easily if needed. Close it all up and you will stay warm and dry in the rain, hail, cold wind and snow. So soft cab enclosures give you options to get your comfort level just right, so is kind of like a HVAC.
Hard lower doors are a great product with its own advantages & if that's what you want and need, that's all good too.