Salt is one of the easiest and cheapest items to store. It stores indefinitely as long as it doesn't get wet. You can usually purchase enough salt for your three-month supply and a bunch (9 months-worth) for your longer-term supply for less than $10! It is often available for $.33 per 1 1/2 pound container. It is also available at Costco in large quantity sacks for a similar price. Make sure that you purchase "Iodized" salt.
Salt is a good preservative, but I would say that taste is the most important reason to store salt. Have you ever had cookies/oatmeal/fill-in-the-blank without salt? It's terrible! Salt, like sugar, makes most things taste better.