There are a number of items that reduce the resource costs of various skills. For example a 20% resource cost reduction to the Ancient Spear skill (Barbarian) would make the skill cost 20 Fury rather than 25 Fury. This seems fairly straight forward.
Ancient Spear can be runed with "Boulder Toss" which then makes the skill spend "all remaining fury to deal 20% weapon damage per fury spent" in an AOE. If you save up your default maximum of 100 Fury and then cast Ancient Spear with the Boulder Toss rune you deal 2000% weapon damage to your target and every enemy unfortunate enough to be standing in his vicinity. Pretty cool and again fairly straight forward. But wait...what happens when you mix the resource cost reduction with the skill that says "spend all the resource"?
Is the resource cost reduction stat ignored?
Does the skill deal more damage?
Is there some other way it works that I haven't thought of?