When you arrive in Ostagar, buy the Backpack from the Quartermaster. Note: You 
should have enough money all long as you collected sell-able items and gear 
during the beginning storyline. Successfully complete the quest where you and 
the other recruits enter the Wilds; make sure you explore the entire map. 
Successfully complete the "Joining" ritual, and become a Grey Warden. Before 
attending the meeting with Duncan and the King, go back to the Quartermaster, 
and there should be another Backpack available to purchase at the same price as 
before.