I have upgraded a few floors but only a few times. It seems to get rather expensive quickly in towerbux, but how much does inventory increase per rank? Is there a formula that I can adhere to?
Inventory increases by 10% of the base amount per upgrade rank.
That is, if at rank 1 a non-dream-job worker would stock 100 of an item, every upgrade rank increases the amount they stock by 10. Thus at rank two they'll stock 110, at rank three 120, at rank four 130, etc. The dream job bonus is applied after rank bonus, so if that was the worker's dream job they'd stock 200 at rank one, 220 at rank two, 240 at rank three, etc.
It's generally best to focus on the stores that have the most inventory (after factoring in dream job bonuses).