If DRL's use a full mpg, heaven forbid using low beams, and bend over to Big Oil if you're cruising rural america with high beams on.
All kidding aside, GM estimates costs of $3 per year, Transport Canada $3-$15 for dedicated DRL Systems and $40 for reduced intensity low beams.
The Transport Canada estimates include bulb replacement costs, a task I have yet to do myself or see my parents do while owning DRL vehicles.
http://hldi.org/research/qanda/drl.html#5