Late Filing Penalties
The filing deadlines for Personal Income Tax is April 30th annually and for anyone with a small business (not incorporated) the deadline is June 15th annually.
Some information from Canada Revenue Agency:
Late filing penalties can add up. The penalty is 5% of your 2015 balance owing, plus 1% of your balance owing for each full month your return is late, to a maximum of 12 months. If the CRA charged a late-filing penalty on your return for 2012, 2013, or 2014 your late-filing penalty for 2015 may be 10% of your 2015 balance owing, plus 2% of your 2015 balance owing for each full month your return is late, to a maximum of 20 months.
Further penalties may be assessed for repeat offenders and other violations. Yes it can be quite expensive.
More information can be found on the CRA Website Interest and Penalties.