If you're talking about what I think you're talking about:
Suppose that the Xth social policy has a base marginal cost of Y.
The actual marginal cost of the Xth policy is Y(1+0.15*(N-1)) where N is the number of cities you have.
That is, each new city you have makes policies 15% more expensive.
With the policy that reduces this, the actual marginal cost of the Xth policy is Y(1+0.1*(N-1)).
So the policy reduces the coefficient on the number of cities by one third, from 0.15 down to 0.1.
[Note, these used to be 0.3 and 0.15 before the vanilla patch change.]