Why are the 20x21x1 $73? That definitely not a deal.
The 20x20x1 are $25.
My size is $1 cheaper than Amazon. Much cheaper options available.
WOW, my odd size is available! (20x22x1)
I prefer to stick with MERV-11 though…
Supply vs demand? Maybe that size isn’t as popular (e.g. not many made).
HVAC ducts come pretty much in standard sizes, so the majority of systems will use probably 4 filter sizes. The standard sizes such a 20x25 are much cheaper because the sell a lot of them. Th odd sizes are not sold much, so the price is much higher, almost like they don’t want to sell them but feel they need to have all the sizes displayed for show and tell.