Based on a $100,000 mortgage at 6.00% interest for a 5-year term amortized over 25 years.
Your montly payment would be $639.81.
You would pay $28,225.07 in interest over the first 5 years.
You would pay $10,163.50 in principle over the same 5 years.
Principle Balance left owing after 5 years: $89,836.47
If you paid an extra $50 per month, each month for 5 years.
You would save $388.34 in interest
You would pay the principle down by $3,388.34
If you continued this process every year you would save 18 months worth of payments; 1 1/2 year's worth of mortgage payments. At $639.81 per mortage payment that means a savings of $11,516.58
p.s. Another idea to consider is to round up your payments. Sometimes for a budget it is easier to remember that your monthly mortgage payment is $900 than $865.81. If you can afford to pay the $900 then talk to your financial institution about rounding up your payments.