I am looking particularly at the Dead Thrall spell.. on the wiki it says base cost 1,000. But my character currently has no lower magic cost gear on and the cost for me is 593. Does my skill in the particular school of magic lower the cost even without taking the lower cost perks?

Yes. Cost reduction is about 41% at skill 100. Base cost is the cost when the skill level is 0. This is true for the other schools of magic as well. Increasing skill gradually reduces cost of the spells.

Thanks!