So if you have an 500 Minute Canada-Wide calling plan, an 5-minute incoming call from the US will count as 5 minutes out of your 500. If you're on an Unlimited Canada-Wide Calling plan, incoming calls from the US don't use up your minutes.
The only time you're billed anything extra is if you're calling from Canada to the US, which is 50cents a minute, or $10/mo for unlimited.
I don't believe there is an unlimited incoming US calling plan (EDIT-at least 1 that you can add-on if you don't already have Unlimited CW calling)
Click on the monthly add-ons:
If you are using your cell to answer business calls from the US, it will be more attractive to your callers to obtain a local US or 800-number from a VoIP service and get those calls forwarded to your Canadian phone. Most US callers are accustomed to nationwide calling being free and will balk at calling a foreign Canadian number.