The 42" size is more rarely done, and when things aren't built and sold in large quantities, they cost more. I think that 42" size isn't as popular because as you pointed out, you don't get a lot more cu. ft. with that 6" additional width.

But I also wanted to tell you that there are several GE 42" fridges that are $6,700 to $7.400. Still expensive, but a lot less than the $9,000 you mentioned.

https://www.ajmadison.com/cgi-bin/ajmadison/CSB42WSKSS.html

The price difference in a subzero 36" vs 42" is about $500

Thanks for your feedback. Smm5525 I was thinking more like a 36 inch Samsung etc which is much less costly and the same interior space.

36" is the biggest standard size and will be the best bang for buck size. Anything bigger is a built in style and substantially more expensive. It's a different product. The difference in price is more due to difference in style rather than difference in size and if you were to compare a built in 36 vs 42" by same brand the price difference would be far less (with both way more than a standard or counter depth fridge). Like many things in life what you get for what you pay doesn't have a linear relationship.

I haven't checked but if the Samsung comes in 42" it might be a built in vs freestanding

It sounds like you're looking at a built-in versus a freestanding model - different space constraints and installs. A 36" freestanding model can have a similar capacity to a 42" built-in, because with the built-in you're losing that extra depth and making it up slightly in height. 42" is pretty much always built-in, and stands taller with options starting around $7000, but freestanding is most commonly 36" wide and ranges in price from like $1000-$4000.