Tested in a no-CP environment, with no other skill, racial, or set-based forms of sprint cost reduction.

Naked sprint cost: 546 (the normal base cost)
5pcs light armor sprint cost: 546

The tooltip indicates there should be a 15% cost decrease (3% per pc of light) but this is entirely non-functional at the moment.

Also tested in a CP environment to confirm that the CP star "Sprinter" is working fine in this regard.
Edit: Also tested the Orc racial "Swift Warrior" passive to confirm it is working.