Hi Adriana,
The whole point of an SSL certificate is so that the browser has a reasonable degree of trust in the server's public key for HTTPS transactions. As per my opinion SSL has advantages for SEO and also for creating trust with the website visitors.
Do the SSL certificate actually matter, you should read this study / research http://blog.searchmetrics.com/us/2015/03/03/https-vs-http-website-ssl-tls-encryption-ranking-seo-secure-connection/
Generally, browsers come pre-loaded with lists of people/organizations that the browser should trust. When your website presents this certificate to a browser and the browser would unquestioningly accept it (assuming that you've paid a sufficiently well-known CA (certifying authority ) that is likely to be in the trusted lists of all browsers).
Now, it's the trust level of these CA which matters to the browsers and search engines, then there are risks / disadvantages of using free certificates. Check following links
https://www.cybersecureasia.com/blog/the-risk-in-free-ssl-certificate
https://www.quora.com/What-are-the-pros-and-cons-using-a-free-SSL-Certificate-for-signing-and-encrypting-email
https://www.volusion.com/ecommerce-blog/articles/5-reasons-you-should-beware-of-free-ssl-certificates/
Coming back to original question, whether free vs paid SSL would matter for SEO, on the surface, it looks it won't matter if your CA has enough trust with browsers, however, if you scratch the surface there are some hidden facts which are highlighted in the disadvantages of a Free SSL mentioned on above links.
It would be better to get a paid certificate, be safe and secure without being worried about if it would hurt your SEO.
I hope this helps, let me know if you have further questions via your response.
Regards,
Vijay