How much you need to reduce costs, in order to double your profit, is dependent upon your current costs and revenue. So you don't necessarily need to eliminate all costs to double profit.

Suppose you purchase widgets from a manufacturer for 100$ and sell them for 101$. In that case, 101$(revenue per widget) - 100$(cost per widget) = 1$(profit per widget). Suppose you then change manufacturers and reduce your cost per widget to 99$. You now make 2$ profit per widget. By reducing costs by 1%, you doubled your profits. Much easier than doubling your sales.