I don't see why you doubt a priori that there is a cost saving here. A high pole is likely much more expensive than a low one since it needs a much higher stiffness in order to prevent it from swinging all the time. High poles may be a costly necessity to allow the use of high-efficiency, high-power HPS lamps and to prevent dazzling road users with bright light sources close to the horizon.
LED lamps are much easier to design for a specific light distribution than HPS lamps, so they could have kept the same number of high poles if that had been cheaper. I'm sure that the road planners actually do a cost analysis.