Don't take this too seriously, it was just a thought experiment I entertained myself with the other day (1/2002) while walking.

Which is more cost efficient?

If we take efficiency to mean perimeter/area. Then...
Given a circle of radius r
Area = [pi] * r**2; Perimeter = 2 * [pi] * r;
Take a square of side 2 * r
(Thusly, the circle fits neatly into the square, and they are near the same size.)
Area = 4 * r**2; Perimeter = 8 * r;
The efficiency of the square is
8 * r / 4 * r**2 = 2 / r;
The efficiency of the circle is
2 * [pi] * r / [pi] * r**2 = 2 / r;
The difference of these efficiencies is zero
The two shapes are equally efficient.