Most of my previous post refer to Social Media presence and posting as a brand / persona. For the most part the only cost is time and the number one main is to build relationships with your customers, existing or potential.
Social Media Marketing can also come in the form of paid advertising. Ranging from promoted Tweets to paid ads on Facebook this can be a very effective way of reaching more customers.
You need to understand where your audience is, so channels such as LinkedIn, Google+ and others may be very relevant for you.