This means that you will hit 100% of households once, and that it will be optional for 30% of households.
Some witnesses of the conservative persuasion appeared before the committee. They were quite shocked by the compulsory nature of the long-form census and wanted it to disappear. However, when questioned, we realized that none had ever received it. They even said quite clearly that they would not fill it. This means that those people will not respond to the voluntary questionnaire. They said it was an infringement of their privacy, on top of many other reasons.
How much does it cost to encourage Canadians to fill the voluntary questionnaire? Since people will not be forced to fill it, they might be less tempted to do so. You will therefore have to spend some money to inform Canadians and to encourage them to fill the questionnaire if you want to collect useful data.