True/False: air gap length and cost

TRUE.

The air gap needs most of the field ampere-turns ($AT_g = 800000\,B_g K_g l_g$). A longer gap therefore needs more field mmf, which means more field copper, larger field coils and poles, and a larger frame. Field copper loss also rises. All these increase the cost of the machine. (A longer gap does reduce armature reaction and noise, but it is kept just long enough to limit armature reaction because of this extra cost.)

Example: doubling $l_g$ from 3 mm to 6 mm roughly doubles the gap ampere-turns, which form the largest part of the field mmf per pole, so field copper and field loss rise almost in the same proportion. So the gap is not made longer than needed: in practice the gap ampere-turns are kept at about 0.5–0.7 times the armature ampere-turns per pole, just enough to control armature reaction.