From what I've read elsewhere it costs in the £500-1000 range to upgrade from economy to business on board if flying long haul. Might once have been considered a 'bargain', but BA has significantly cut the prices of it's business class seats over the past few years and it probably isn't such a good deal anymore. Haven't heard the cost if flying short haul, but if short haul I cannot see the point of upgrading on board unless it was was something like £20, you would mainly have to put a value on the additional booze/food, cannot be worth much. IMO of course, others might have a different valuation method, people are funny like that
Those who study this sort of thing seem to say the best value way to get into business class on BA is to buy a WT+ ticket and then use miles to upgrade. Or get somebody else to pay, that's what the mrs does