When you boost a post on Facebook, you can capitalize on your money spent by going back in and inviting everyone who liked the post to also like your page. It is likely relevant content and this has a high conversion rate. If you promote the page via Facebook to get likes, it can cost at least .25 cents per likes. My experience has been that a large number of people will accept the invite to like your page. It is a tedious task, but well worth your time to do it! This is a good way to get more likes on your page, and it costs nothing extra to do it!