Table salt would be a really bad idea! Aquarium salt is the best bet, ofcourse... if it wasnt then there would be a lot of sued Aquarium salt companies. However, if you want a cheap alternative, you could probably buy sea salt at your local store or supermarket. It tastes better in soups and usually has no added Iodine... be sure to read the label though.
Please note that I use neither sea salt nor aquarium salt, Im just guessing that the sea salt would be cheaper considering the fact that most aquarium related products tend to be pretty overpriced (filter sponges / charcoal especially).
BTW, I thought I read somewhere that Mollies go in freshwater as well as brackish and sea water. So, do you really need to add the salt at all?