I've bought alot of accessories, mostly from
e-Bay, and it really depends on what you buy and from whom.
From what I remember, the Duty part is nil. But you do pay the brokers to bring it into the country as well as the GST + PST.
If you buy 1 or 2 smaller accessories from 1 seller, usually you can get the seller to put on the Customs Declaration a value of $20, therefore not costing any extra at the border.
The best places to buy from will ship USPS, then the package goes through Canada Post, who only charges $5 as a handling/brokerage fee. Companies like UPS/FedEx usually charge $50 or more just to bring it into Canada!
Trust me, no whatter what you pay for Customs/Etc. it's still WAY cheaper that any local Stealership
Plus I find a WAY BETTER selection of items in the US.