There's no such thing as zero price, as people would have to be hired to post messages, along with whatever equipment they use to do it. For example, the USA uses multiple paid trolls, disguised private networks, and special software to co-ordinate and tag-team propaganda posts in just one social media item, like a single article. Over a year, just that would be several hundred thousand.
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$100,000 would probably hire just 2 people to post comments for a year. I doubt that would make much impact. The USA contracts entire companies to pay people to post propaganda in social media.
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https://www.theguardian.com/technology/2011/mar/17/us-spy-operation-social-networks
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https://www.theguardian.com/commentisfree/cifamerica/2011/mar/17/us-internet-morals-clumsy-spammer
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http://www.reuters.com/article/us-election-intelligence-commentary-idUSKCN10F1H5
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http://www.businessinsider.com/ndaa-legalizes-propaganda-2012-5
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https://consortiumnews.com/2017/03/25/how-us-flooded-the-world-with-psyops/