99.9% there won't be any duty, but you still have to pay GST and PST (or HST...). 
And then you have to pay canada post $5 ($8 on express stuff) for the honour of taking your tax.. and for UPS it starts at $40 IIRC. fuck them.
With govn't mail, it's kind of hit and miss whether they ding you or not. I guess it depends what mood the customs dude is in. There's been times where I haven't had to pay on $400 items, and times where I've had to pay on $15 dollar stuff. They generally don't ding you on stuff in the sub $50 range though, IME.
So - in ontario (your profile)
$100 item
+ $5 GST
+ $8 PST
+ $5 tax handling whatever rape charge
= $118.
but if they only ding it every second time, average it out to say $109.