i'm gonna calculate by the area of pizza to see how much food you get for 10 and 20 dollars
A...area
r...radius
d...diameter
d1=12 r1=6
d2=18 r2=9
A1=r1^2×3.14
A1=6^2×3.14
A1=36×3.14
A1=113.04
A2=r2^2×3.13
A2=9^2×3.14
A2=81×3.14
A2=254.34
if you bought 2, 12 diameter pizzas for 20 dollars you would get 226.08 area od food,still less if you bought 1, 18 diameter pizza for 20 dollars. because 223.08 < 254.34