SX 70 image area 7.9 x 7.9 = 62.41 cm(2)
Fuji instax image area = 4.6 x 6.2 = 28.52 cm(2)

SX 70: price per image/cm(2) = 2.5/62.41 = 0.040
Fuji instax: price per image/cm(2) = 1/28.52 = 0.035


So the price difference between the two image format is not that far apart.