When is Manage By Booking Promotion Upgrade not a promotion?

My return booking from Lisbon to Heathrow on Friday evening is currently in EuroTraveller. When I access the upgrade option in "manage by booking" it says "would you like to upgrade for €98". However, when I select this option and go through to the payment option, the total to pay is €481! I have tried it several times and the same happens. The offer is not "from" but "for". I even called BA customer services to be told that probably the issue is other flights on Friday from Lisbon are offering €98 so the booking system is "confused"!! Rather odd to me but hey ho, but weird that the computer can be confused!