T-Mobile To Go prepaid does have roaming in Canada.

For the original poster, a T-Mobile To Go plan would cost, assuming the purchase of $100 refills that give airtime at $0.10 per minute:

100 minutes of calls to Canada: $0.20 or $0.50 per minute extra for $0.30 or $0.60 per minute = $30 or $60 (T-Mobile's web site has two pages which say different things: http://www.t-mobile.com/Internationa...eOverview.aspx and http://www.t-mobile.com/shop/plans/prepaidrates.aspx )
100 minutes of calls within the US: $0.10 per minute = $10
50 texts: $0.10 to send, $0.05 to receive in the US or Canada = $2.50 to $5
roaming within Canada: $0.69 additional per minute for voice, text is no extra charge

So about $45 or $75 per month plus whatever roaming charges. However, you can bring the cost down a lot by texting instead of calling to or within Canada.

Of course, there may be other plans, either prepaid or postpaid, which have lower total costs for the usage pattern described (however, many prepaid plans do not have roaming to Canada).

How much does your Canadian cell phone plan cost to roam in the US or call the US? Depending on that, it may be worth keeping a Canadian plan as well as a US plan.