When I was a kid, my mom, with five kids, often tried to find ways to cut costs without cutting quality. One thing she tried was making chocolate chip cookies with shortening and margarine, which are both cheaper than butter. They were passable, and, because she had always used those ingredients, we didn't know what we were missing. I do, however, remember seeing puddles of oil that came out of the dough the few times she tried using corn oil margarine. it was very unappetizing!
When I started baking for myself, I was interested in experimenting. I tried all kinds of different things. I used shortening, all the different varieties of margarine (including the one that claims to mimic butter's taste), and even butter-flavored shortening (I don't recommend it). But when I used butter, I knew that I would keep using it. The taste was so far above all the rest that there was no contest. It didn't leave a greasy slick on my palate, the way shortening did. The aroma that filled my house was better than any incense or air-freshener. I stopped trying to save money on it, and bought better and better quality butter. I found that unsalted butter was the best kind to use -- I could control the amount of salt in the recipe better, and, since salt is hygroscopic (absorbs water), the moisture level as well.
When you bake, you are investing your time and effort as well as money in the product. Why not make the most of it? If you calculate the true cost of baking, including the cost of your time (and isn't your time precious?), you'll see that the cost of ingredients is often the smallest part of the total. Use the best ingredients you can afford; in terms of butter vs. substitutes, that will almost always be butter.
Butter freezes beautifully; we recommend storing it in the freezer, as there is less likelihood of strong odors (such as onions, garlic or spices) infiltrating your butter there. Rotate your stock. If you don't bake often, buy it fresh and store it briefly in the refrigerator. When you are planning on baking a cake, we recommend taking the butter out at least an hour ahead of time to soften to room temperature (two hours might be necessary in the winter). Some bakers advise a temperature of 65˚-68˚ for your butter; you don't have to make a Spielbergian production of it -- just make sure the butter is soft to the touch, but not liquid. You can use the microwave to soften the butter, but do this very carefully -- use the lowest power setting, and go for 30 seconds at a time so that you don't melt it. If you do melt it, and you've been foresighted enough to put it in a dish so it isn't all over the microwave tray, put it in the refrigerator and use it for something that calls for melted butter.