Well you do already have XP requirements that can take a casual player more than a decade to reach, and BB still doesnt seem obliged to adjust anything ...
The good thing about those costs is that theyre irrelevant either way since you can already outproduce your needs with lvl 6 buffed buildings without having to bother with further upgrades. Not to mention that since the production increases linearly, subsequent upgrades lose value while the costs increase, so one more reason to give up on that. Unless you want esthethics.