This is an easy problem in theory and there are many unit cost calculations that yield a valid solution:
cost/(PI*r*r)
cost/(r*r)
cost/(d*d)

Calculations that fail are:
cost/d
cost/r
using integer division either as cost/(d*d) or d*d/cost

For the astute, it is possible to use integer calculations throughout
(as shown in pizza.cpp). But the closest price differential of all
tests is between the 10-inch and 12-inch pizzas on the first example.