You actually do the same amount of credit points per year as a single degree and so pay the same per year as a single degree. It's just a double degree is a few years longer.
The combo will also determine the cost - you can't just add an extra year's cost of a single degree : eg a BScience / BCommerce -the commerce courses cost more than twice that of science courses as the latter are considered national priorities (and we don't need any more bankers/economists ).