I will be attending Western for my MA in Classics in Sept. Their funding offer does change considerably when you win a SSHRC/OGS. My original offer was $18,000 (which included a few grand in entrance scholarships, ta ships, and general funding), then when I was awarded an OGS which is $15,000, it went up to $24,000 ($15,000 plus roughly $9,000 TAship). I was told if i win a SSHRC my total funding would go up to $28,000. I think Western's Factulty of Graduate Studies has a fairly standardized funding package, so yours should be something similar.
It obviously goes up quite a bit, but they do 'clawback' some of the funding. I'm pinching myself too! $28,000 to stay in school? YES PLEASE!