The way I look at it (now being an occasional flyer rather than spending too many Friday evenings in airports waiting for a delayed flight), is how does it compare to me paying for the flight in Y, as that is the amount I would at least be paying in any case for any flights.
As others have pointed out, the taxes and fuel surcharges for flying in premium cabins is now approaching this. For example my 2-4-1 this year is to SIN in F and the taxes etc were approx £1200 - which is probably similar to what you would pay for 2 people to fly Y. I therefore see it as good value, as I would be spending that money in any case.

In true BA Board fashion though Mrs Toothy and I will be trying to drink as much of that £1200 worth whilst in the lounge / on board

As BAHumbug said though, I am surprised BA haven't added an extra option of paying for the taxes etc with extra avios. It would be a way of reducing down the number of outstanding miles for those who have a large mile balance.

T