I have a bit of a brain-teasing exercise on my hands which I am battling to solve, and I was wondering if there are any mathematical / Excel fundis that can assist me?

One of my clients purchases a set of gift boxes at a particular cost and needs to work out a formula to cost each of the individual boxes. There are 15 boxes in the set but, here's the problem, the boxes are not the same size.

The 15 boxes range from the smallest box, which then fits into the next size box and so on. I would say that the percentage increase in size from box 1 to box 2 is the same as from box 2 to box 3, etc.

Ideally I would like a formula or a model that can be set up on Excel, so that the client can simply insert the cost of the entire set and then let the model calculate the cost per individual box.