I got an upgrade offer a few years back of GBP79 on LHR - ZUR for an upgrade to Club Europe, which I took as the wife had so much baggage were looking at more than that if we went economy and paid the excess.
Prices vary greatly for upgrades, I once spent US50 to upgrade on LAX - Phoenix which was offered when I swiped my pass at the gate. In this case you would only get the cabin benefit as you're too late for the lounge and bags are already in the hold.
Premium economy cabins are relatively small so the chance of being upgraded from PE into business is quite good if you have the status and they're overbooked.
I have heard that if you regularly upgrade, the airlines deliberately might not offer you the option on every flight as they feel you are a business class passenger and should buy a normal business class ticket.
