I have seen the upgrade price vary.
My colleague found that the price to upgrade from IST to LHR varied from, IIRC, GBP150 to several hundreds of GBPs, to not available and then back to GBP150 just before the flight.
Unfortunately, as my Y trips are for work and booked by the Corporate Travel Agent, I do not benefit from these MMB offers until OLCI when they no longer exist or are very expensive.