DIY Plug-in- make your own by refilling an empty plug in bulb with about 20 drops of essential oil, topped with water. Get inexpensive essential oils for about $4 for 1oz. bottle and it makes it about $0.33 per refill.
DIY wall flowers using body spray as a cheap alternative to refill and reuse the leftover bulb. A more natural method would be to wash the bulb, fill it up with water, then add the essential oils of your choice to create a scent of your own.