The short answer is no - there is no reliable pattern. The upgrade prices are opportunistically driven by the revenue mangement system trying to maximize the overall revenue generated on the flight.

As such, they are constantly adjusted to reflect the current load predictions in both cabins. When the booking count changes, so does the upgrade price. Furthermore, I assume that the price also takes into account your existing fare base.

Either way, why would you want to upgrade a 90min LHR-ZRH flight, where you get a small snack and sit in the same seat as in the back? Lounge? Extra bags? (I am not sure if buying an upgrade would change your baggage allowance...)