wait wait wait............ I-94 states clearly you can keep the card if you are going to Canada as long as the I94 does not expire............
In general, if you have been admitted to the United States under most visa classifications if you take a short trip (30 days or less) to Canada or Mexico, you may retain your I-94/I-94W, so that when you resume your visit to the United States you are readmitted for the balance of the time remaining on your I-94/I-94W.
http://www.cbp.gov/xp/cgov/travel/id...ng_out_i94.xml
Yes, you need to go through the entire application process and pay $6.95 (if processed by land)