They are certainly rules but we don't know them.
One rule for sure is that when paid upgrades are proposed, they are not proposed to every pax of the concerned flight (it is targeted) and when a pax is targeted and if you fly a lot, you are not supposed to receive this offer for every flight. the principle is that you cannot be sure to receive this offer, so if you want to be sure to fly the upper class, you book it.
As there is no relationship between price of the upgrade and fare paid and route length; I suspect it's the revenue management which determines the price for a specific flight, based on actual bookings, number of seats proposed for the paid upgrade, etc