I think she may not have understood that you want a 2-tier Mad Hatter Cake. Officially, the standard Mad Hatter cake is 3 tiers, and it is $980 (all the prices I'm giving you are from the most recent cake pricing sheet). Any other number of tiers is the exception, so you need to specify that or they'll think you want the 3-tier.
So you would pay the difference between the credit they give you for the standard Escape cake and the $450 2-tier Mad Hatter, plus tax and service charge. It's interesting that, from what she told you, they will double the standard credit to apply to the pricier cake... scratchin'