First of all- what exactly are evaporated milk and sweetened condensed milk? Both are shelf-stable types of whole milk where half of the water in the milk has been removed, resulting in a much richer product. Condensed milk has lots of added sugar, thus adding the term, sweetened.
I've seen various recipes for making evaporated milk using powdered milk, however, I can't imagine it would have the same qualities being as powdered milk is non-fat and really, the benefits of evaporated milk come as a result of the fat included! For that reason, we feel like evaporated milk, though not as necessary as powdered milk, is still an important part of food storage.
Due to the added sugar, condensed milk is mainly used in desserts. This type of milk is certainly the least essential to have on hand, however, I use it often enough and plan to have several cans in my storage. If something were to happen that required my family to live exclusively out of our food storage, I'm sure they'd appreciate of my idea to store dessert items!
To make condensed milk from evaporated milk, simply heat the milk slowly and add 1 1/4 cups of sugar. Since condensed milk is more expensive than evaporated milk, it might be a good idea to store just a few cans, and then make it using evaporated milk as needed.
Evaporated milk, on the other hand, is very useful! Evaporated milk used to only be made with full-fat whole milk. However, there are now low fat and non-fat versions sold everywhere. It is important to note that without the fat, these versions won't work very well when you're relying on the fat to add creaminess to your food!
Also, you can add water to evaporated milk and the result is whole milk, which is very beneficial to have in your storage if you have toddlers in your family. Just add 1 can of water to a normal 12-oz can of evaporated milk and chill, just as you would normal milk.
Since it's essentially a very rich milk, I substitute it for cream in a variety of recipes. Cream is expensive to buy and often time you only need a small amount in recipes, so instead, just use evaporated milk. We can hardly tell the difference and I love not having to spend the extra money.
To give you an idea of just what evaporated milk can do, check out this incredible chocolate mousse cake Brandy from The Prudent Homemaker made with it! She just substituted evaporated milk for the heavy whipping cream and voila! An easier version of the recipe she used can be found here. She also has several recipes on her site featuring evaporated milk, including Chicken Enchiladas, fudge, and Tomato Basil Soup.
I've gotten evaporated milk for as little as .24 a can, however, sales like this are not common. I just bought a case at Smith's and paid .65 a can, which is a decent deal.