Cost effective? - Utilizing existing space within the thermal envelope of the house, possibly built in shelving, under stairs, kneewall space. Depends on what you are storing, how visible you would like it to be and the overall layout of your house.
Bathroom essentials: The room you wake up first to in the morning and see late at night often has the worst storage. Your bathroom can be maximized by assessing who uses the bathroom and for what. Shared bathrooms with kids can utilize open shelving and baskets under the sink and behind the door can house small electrical beauty appliances when not in use. A shoe organizer can turn into a life-saving space saver that holds makeup, combs & brushes, jewelry and even accessories such as scarves and belts.