I have a bride who wants her 8, 12, and 16 tiered cake to be deep layers. I am having trouble coming up with servings with this extra layer being added. Also, if I am doing 6" layers vs. 4" should I dowels and boards between every 3 layers. Thank you. 

there are two ways of thinking of this:  Does the bride want extra tall pieces of cake, say a 1" x 2" x 6" piece of cake--12 cubic inches of cake, or 50% more than the standard 8 cubic inch in a 1x2x4.  (which you will have to advise her that she will have to make sure that the venue is going to have salad or dinner plates on which to cut and serve the cake, because they will not fit on a standard cake plate), or does she want to have the look of the extra tall, double-barreled design where there are two 4" high tiers of the same diameter stacked with dowels and a board and finished to look like one tier.   

What size are your standard tiers? If you figure out your price per cubic inch, then you can figure out how much more you should charge for a layer that is 50% more. When calculating servings with the extra height, it doesn't mean an extra serving, just a larger serving. For example, if your 8 cubic inch slice is $4 a serving, that is .50 a cubic inch. A cake that is 5" high (or 10 cubic inches), then you would charge $5 a serving. 

AThank you for your feedback. The bride wants extra tall (6") cake layers. Her tier is 8", 12", 16". I will need to do your cubic inch math.

AThat is a huge piece of cake! You might want to do a 3D mock up out of paper to illustrate to her how big it is and what it looks like on a plate. Also construct it in a way with sturdy fillings so that the event staff can get those big slices on the plate. I see disaster if they try to do a circle cut on something that high-- that final core is going to topple being that high and narrow.