I can usually buy them cheaply with coupons (especially at Costco).
Two make the sponge last twice as long, I cut it in half with a sharp pair of scissors.
One half of the sponge is just the right size (especially for kid's size hands) to wash dirty dishes. I always place my sponges on the top rack of the dishwasher when I wash a load of dishes in it to disinfect them.
You need to keep sponges dry between uses, so that they don't become a breeding ground for bacteria. An old plastic berry basket in the drying rack works great for hold drying sponges (and the lids to water bottles, so that they don't don't fall down the garbage disposal).
This little trick my not save me a ton of money, but every little bit helps!