Of course they can't. It is doubtful if the other two games will cost more than expansions. As far as I remember, expansions normally cost $35-$40 upon release.
ON what grounds would Blizzard charge $60 per expansion? The fact is that the engine is already created, as are most of the units, etc. The framework is set. The expansions are merely another campaign with a few extra units and probably some more features.
As for the complaints about one campaign, don't forget that you get about 30 missions per campaign. That equals the same amount of missions as all three races in the original StarCraft, as each race had about 10.