Coming to SEM, i.e. paid ads, they may generate revenue upto some extent but not large as when compared to results given by SEO+SMO.
Consider if you promote good quality content then it is called SMO stunt which gives you free publicity.
Then think about some one "Queries Google - for your respective business", then most of the people chose SERP of page #1 rather than other pages.
If you did not get into page#1 then you may lose many deals.
So, you must do SEO and then SMO to elevate your business standards.
Though SEO is unpredictable, but SMO is the great assisting source for SEO.