The headline example
Take a $450,000 mortgage amortized over 25 years with monthly payments. At 4.55%, the estimated payment is about $2,503. At 4.75%, it is about $2,554. That is a difference of roughly $50 a month.
Over a five-year term of 60 payments, the gap adds up to about $3,024 in additional payments. The interest cost is larger than that. Because a higher rate means less of each payment goes to principal, the extra interest paid over those five years comes to about $4,315, and the balance left at the end of the term is about $1,290 higher.
These figures use semi-annual compounding, the convention for Canadian fixed-rate mortgages. As a scale check, the Financial Consumer Agency of Canada's amortization example shows a $300,000 mortgage over 25 years costing $1,660.42 a month at 4.5% and $1,710.82 at 4.8%, a difference of $50.40 for a 30-basis-point gap.
Find your balance
The effect scales with the size of the mortgage. Roughly, each $100,000 of balance adds about $11 a month for every 20 basis points.
Assumptions: 25-year amortization, monthly payments, semi-annual compounding. Figures are Homeowner.ca estimates rounded to the nearest dollar, and exclude property tax, mortgage default insurance premiums and lender fees. The 4.55% and 4.75% rates are illustrative, not quotes from CIBC or TD. Your lender's calculation may differ.
None of these numbers is dramatic on its own. The point is that they are real, they repeat every month for the full term, and they arrive without any change in your income or your home.