Google released in 2014 that there was a very slight ranking increase for some sites (less than 1%), however given that Google has expressed a desire to see all sites serving pages securely it may become a ranking factor in the future. But it is worthwhile, mainly to protect personal customer data, increase customer trust and satisfaction (especially since browsers are starting to highlight insecure connections), and resolve duplicates among other reasons covered in other similar articles.
As of 2015, Googlebot automatically began checking as to whether each link could be viewed over https, and if so would index the https version site links. Any site link with https gets a very slight boost increase.
Just keep in mind that there will be some work to migrate your site to SSL, depending on how it and the server is setup (absolute links, plugins that may not be coded correctly to handle SSL gracefully, etc.). But resolving these things will generally result in a better site all round.
A no cost way of activating SSL is by using Cloudflare. Their free service provides SSL automatically using their own trusted certificates. I don't work for them but have successfully used their service for years. Our sites are better protected, served faster, and more secure because of it. So as to avoid being seen as promoting them, you should check them out for yourself. They provide many other benefits (proxying, CDN, offline serving, enhanced security) as well.
Google has more recently revealed that https is going to be a more important factor in 2017, particularly for site credibility. Whether your site is considered "trustworthy" is of significant importance to your site visitors - for that reason alone SSL is now becoming a "must do", particularly with more recent browser versions have moved to more explicitly highlight sites which are not being served securely, with users warned that they are accessing insecure form pages.
For the benefit of subsequent readers, Let's Encrypt is a free certificate authority who launched in 2016, launched by the EFF.