With that logic fares at Heathrow are low and airlines are paying mentally high prices to offer basement fares? It's not that simple of course, the front end are cross-subsidising the back end and there's more front end in London remember. This is part of the reason regional direct flying is more expensive in economy.
Obviously my point was very much black and white and in what is known as a 'perfect' economy.
Your point is very much valid also and goes to show that there are tens of different factors involved however, EK offer the same A380 on LHR and MAN routes yet fares are higher from MAN...