If I decide to move off campus for my second semester, Im only expected to pay for half the rez costs right?

NO! I believe you will still have to pay your residence fees. There was a certain date that you would have had to withdraw from residence in order to get payment back (refunded). If you do withdraw from residence the website states that you will be put on a 'refund waiting list'. Therefore - you only get money back if they were able to find a student to fill your spot.

http://housing.mcmaster.ca/Residence...ithdrawal.html