That proposed formula misses the critical aspect of satisfaction with the game.
It is actually more like...
Time to Get Item = Constant / New Content Required
Constant / Time to Get Item = Player Satisfaction with feeling of progression occuring
Time to Get Item / Constant = Player Satisfaction with feeling of actually being rewarded for gameplay and things not being 'trivial'
Now the key is to maximize the satisfaction that players feel while also minimizing the amount of new content being required. This requires some consideration of how valuable player satisfaction is compared to cost of new content. These things would be modeled by actual survey of players to find the real constants (or logarithmic function that occurs here instead of simple divisions which is of course too simple) with proper weighting factors put in based upon the actual player satisfaction numbers to revenue and new content to cost numbers.
If you do this sort of thing correctly you end up maximizing returns. Good companies do this sort of thing whenever considering important decisions which require non trivial investments (ie new content).