Don't know about the USA, but for Canada there is a list of categories of items with % of duty applied. For example, clothing is 18%. On top of that 18% one would have to add taxes and sometimes a brokering fee of about $5 if it's handled by Canada Post.

BUT the truth is that which package gets intercepted, assessed, and billed is pretty much random. Since in Vancouver, I have been assessed customs to pay on most packages coming from abroad, and duty as applicable. In Montreal, it was 1/2. I guess different points of entry have different thresholds.

So on the US Customs web site there must be a list of duty rates somewhere. In my opinion, you can't count on NOT being assessed fees... so that if you aren't you can celebrate.

[Around here, it gets quite complicated if you need to return the items. You can fill a paper and send it out to have your duty and taxes refunded to you... but I did that two times and never heard back from the Customs Agency.]