It's around $1 to get it over the border to the US post office. Of course you have to pay for whatever the US postage is on top of that.
I print my own US postage labels at home. Their rates are way less than ours. It's around $3-4 for first class up to one pound. Media mail (books, music) is around $4-5 for a couple of pounds. Priority in small boxes and packs is about half of our rates.
ChitChats Express is one company that provides the trucking service. They have locations around the GTA, BC and NS. Another company is Cross Border Pickups in Mississauga. Google for details.
P.S. CC also resells Can Post for slightly less, so if you ship a lot within Canada you can save a few bucks on every package. (Kind of like Public Mobile reselling Telus phone service for cheaper.)