Considering that the value of one of BA's 767s is currently around the US$10m mark - due in large part to their being about the only airline using RB211s on them (a la DC10-40) the ascribed value of US$50m each is a trifle, er, over-optimistic!
The RAF prefers the A330 because they want to have wing drogues as well as body ones - the 767 apparently cannot acommodate this without seriously expensive wing mods.