Maybe your discussion is not only academic, and must be considered in Britain, but as a captain (although based in a 'colony'...), I have never let the cost factor influence my decisions on when and whether to divert-and never will, either. Not if I'm responsible for the ultimate safe end to a flight. I've diverted three or four times. If cost were a factor, the 'bean-counters' would share ultimate responsiblity-but that won't happen..
We try to contact Dispatch long before we reach such a fuel quantity and beware of some (US) Dispatchers "Decision Fuel" figures: these do not reflect true, unanticipated real-world factors, such as ATC denying us the planned altitude and direct routing, or whether the divert route is through crowded airspace and/or in an area of strong thunderstorms.
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Let's make sure that the divert airport has an ILS, or something better than a da^^&#d NDB approach-that is one way to get in a very serious corner with no way to fly a 'real' approach or go anywhere else, based on fuel. Some Rocky Mountain airports (Kalispell, MT) can have unforecast fog from a nearby lake, or unforecast snow-with the runway lights covered up, and no alternate fuel planned to return over the mountains to Great Falls.
Maybe all of this is totally irrelevant, because the 'real world' of airline flying only exists north of France
. If safety clearly comes first, without any quibbling, then who cares about the cost?