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As per CRA, 

The adjusted cost base (ACB) is usually the cost of a property plus any expenses to acquire it, such as commissions and legal fees.

You can calculate Capital gain as Proceeds from disposition- Adjusted cost base (ACB- usually the cost of a property plus any expenses to acquire it, such as commissions and legal fees)-Outlays and Expenses (Amounts that you incurred to sell a capital property).  

Outlays and Expenses (Amounts that you incurred to sell a capital property)-The cost of breaking a mortgage would be part of the 'outlays and expenses' when selling a property.

Please visit the article on Reporting the Sale of Your Principal Residence and How do I report the sale of my home (principal residence)?