I DIY my oil change every 6 months, usually it's 4K miles for my wife's, 6K miles for mine, I use synthetic oil, and buy it when there is rebate, so normally it costs around $20 for each car (including filter). Here is the web page I check for oil deals. It may not apply to Canadian sales though. Currently there is a $10 MIR for Quaker State Ultimate Durability synthetic oil.
http://slickdeals.net/forums/showthread.php?t=1532155