As others have said Airport Upgrade prices vary and range from the difference between your fare paid and full fare for the cabin you want and at the other end of the scale a bit of a bargain. Anything other than this, particularly any fixed price menu would devalue the product and no one would buy the normal price.
In my experience if you want to get a cheap upgrade the POUG upgrade offered through BA.COM in MMB are the best deals, you only get offered when the cabin is undersold and again there is absolutely no consistency or pattern in the offer. Expect the offer to appear in MMB after 72 hours from booking - but it can appear anytime up to the day of departure from that point onwards. It is also selective, in fact if you are on the same flight as colleagues you may find that you are not offered and others are.