Larger sizes do not necessarily need more material. It depends how the pattern pieces fit on the cloth, how wide the cloth is and what the "drop" on the cloth-pattern is; there is always wastage on the cloth. Cheap garments can be cheaper because the pattern on the cloth is not matched say for the left and right sleeves. On the other hand going up one size can sometimes cause almost a doubling of the material required if you suddenly can't fit two pattern pieces side-by-side or the length requires an extra "drop" length of material.
Stitching together larger sizes will cause a trivial increase in the time taken to make the garment - it obviously takes longer to stitch a 50in waistband onto a skirt than a 24in one, but most of the assembly time is taken up with fiddling around with darts etc.
I think, for adults' clothes, it's simpler for the manufacturers to go for a single price for all sizes. With the cost of cloth for expensive garments I can understand why some choose to charge more for larger sizes.