It's a question of volume, not just height. A larger base means a taller, wider, and deeper mini, so materials costs go up at the cubic power of base size. Not only that, but depending on the material and casting method, thicker areas can require different handling than thinner areas, to prevent cracking.
On the other hand, 30mm minis have to have smaller fidly bits to look detailed, and small fidly bits can be very hard to cast.
I want a "do whatever's cheapest" option for size.