The chance of finding a block is '1diff shares'/'network difficulty' while staying inside one 'difficulty fortnight'
In theory that is correct, but practically and in real life, it isn't dead accurate, because the following quote is incorrect
The price you pay for shares is directly related to what is called 100% PPS which is based on 'network difficulty'
Nicehash is a marketplace where people place their 'bids' to accurate a certain amount of the hashrate that is available for rent, in the unlikely event that there is more hashrate available than demand for it, nicehash would probably point it to a PPS pool and pay you accordingly, aside from that, the buyers set the price, not the sellers, if you check the average profitability of NH vs BTC PPS pools, you will always see a variation, the last 3 days NH buyers paid 5% more PPS+ pools, in some rare occasions it becomes more, I was able to mine for a few hours (maybe a full day, can't recall) on NH and got like 7 times more profit because someone was paying a ton of money for hashrate to mine some shitcoin.
Also, there is no guarantee that the renting price per hash would go down with the difficulty increase if there are enough opened and funded orders that pay 0.00367/PH/day even after a difficulty increase of 10%, there is no guarantee that you will get a 10% discount on whatever you rent after the diff change, and the same thing is true for the opposite side of the argument.
However, it's worth mentioning that your theory still stands though not perfectly, due to the fact that most buy orders on NH are probably made by bots, after a diff change to the upside, you usually see a few orders that still pay the same per hash as they paid prior to the change (those are probably humans or badly coded bots), the rest of orders will start to adjust accordingly.