When I complained about the increased costs, what I meant was that when I upgrade other building (resource, raw processing, factory), if my production does not increase, my costs stays the same (e.g. production of level 7 = 16, costs = 3000; production level 8 = 16, costs = 3000). Using the same logic, if max sales does not increase, then it should not increase operational costs.
Using this logic could make a little hard code, of course, but once done, it would be more logical. I think it would be hard to do, because seeing the shops from level 50 Research Level point of view, an upgrade level should increase sales in at least one product (educated guess). But not at each Research Level point of view, because sometimes you only have one product or small numbers that don't increase sales, but costs increase. So, you would have to make operational costs dependent on building level and research level, not only on building level.
Well, this is my point of view.
Best regards,
Universo