Short answer: Yes. Luck above 50 gives all your skills a hidden bonus (+2 per every 5 points of luck above 50, up to a max of 100 in a skill). The higher your skill in a particular school of magic, the less it costs to cast spells from that school.
For the longer answer with more detail, read these two links:
http://www.uesp.net/wiki/Oblivion:Luck
http://www.uesp.net/wiki/Oblivion:Magic_Overview#Overview