The Mage Knight Dragons were $25, IIRC. They also had the complication of the large dial base. Allowing for price increases since the MK Dragons were released, and the savings because they don't have the expense of having a dial, I think $25-$30 is a reasonable estimate for the piece.
Of course, that assumes the size is the same. The MK Dragon base measures 3"x6". The tanks measure 5"x8" approximately - and that's the base they'll be using for their Apocolypse Dragon. The tanks went for $35, I think (no one knows how much the convention exclusive Apocolypse Dragon is going to go for, yet).