I'm flying to AMS tomorrow and wanted to upgrade the Y fare to CE for some time. Upgrade fares started off at £100 then steadily increased to £200, £400 and then something crazy like £650. Needless to stay I didn't bite. Low and behold, last week on randomly checking the upgrade price it had reduced to £59 ^ At that price I couldn't refuse!
I have never seen an ex-LHR upgrade fare so low. The cheapest I have seen otherwise is ex-LGW at £49 as Markie mentions.