Advertising a product through social media is the cheapest way to promote a product as you have said. And sometimes it might be free only that it will require little effort. On facebook an advert can go as low as $10 per month and the audience will be target audience.The cheapest way to advertise a product is through social media. There are so many social media that allow free advertising. Though some block free ads but even if they are not free the value worths the price.