For spousal sponsorship, there is no minimum income requirement. CIC request your income information because part of the sponsorship agreement is that you will be able to financially support your sponsored spouse for the three years. They do not require a minimum income for sponsorship, only that they can see that there is a plan in place and resources available for you to be able to fulfill the financial support of your spouse, in the event that they required financial assistance. If your income recently has been low, then you can demonstrate your spouse's funds and include letters of offered support from family supplements to show that you have a plan in place. You need to provide your financial ‘game plan’ on a separate piece of paper (make sure it is headed with the question you are answering and sign and date it).
Put yourself in the position of the CIC officer reading your game plan. Would it seem sensible, workable and properly thought through to you?