Currently the costs of generic military structures are:
barracks : 200 wood, 100 stone; 150 s stable : 200 wood, 50 stone; 120 s elephant stable : 200 wood, 200 stone; 240 s arsenal : 300 wood ; 180 s
As pointed out by @Edwarf in this forum post, the elephant stable is rather expensive compared to the arsenal (both are city-phase structures).
This patch keeps the elephant stable's resource costs unchanged (it is visually larger than the other three military structures), but lowers the building time from 240 s to 180 s.