My advice would be to get quotes for a suitable size cake with all real cake tiers and then compare it to the price of either the same cake or a cake with more/larger tiers for show as you may find there's not that much difference in price.
I do dummy tiers on occasion but to be honest the price difference isn't that great because good quality dummies cost almost as much as a sponge cake ingredients. The cost of the actual cake inside is your wedding cake tiers is negligable really. The largest proportion of the cost of a wedding cake is the cake maker's time - the time it takes to for consultations, the correspondence, design work, shopping, covering the cakes and the largest part on the decoration and all of this is the same regardless of whether the tiers are real cake or dummy.
It's a good way to have a larger cake than you actually need without having loads of cake left over afterwards but it's not always that much cheaper.
HTH
Catherine