PROS
You get a slight SEO boost
It shows visitors that you are serious
It makes hacking more difficult closing some doors hackers use
It is required if you are handling credit card payments on your site
Any secure sites that want to post your content (exa: banner code you provide them) will have issues
Service workers require SSL (An HTML5 technology which one of the features it allows is push notifications).
The referrer header is lost when going from an SSL site to a non SSL site.  It works fine the other way around.  So if you don't have SSL you won't know as much about where your visitors are coming from.
If you have optin forms (whether for member login or for capturing leads) it is a good idea and browsers are starting to throw warnings if you have forms and don't have SSL.

CONS
Slows down page load VERY slightly
Costs money
When it expires it will throw an error message if you don't remove it or renew it (so if you don't use the site and "forget" about it your site would be down.  I've seen this issue too many times).
It needs a dedicated IP which in most cases means more money and time to setup

NOTES
It's getting cheaper and easier to do SSL than it was in the past.  In the future you may see more and more penalties for not having SSL.  Below is the error I see in the console when visiting a page that has a password field and no SSL:
Password fields present in a form with an insecure (http://) form action. This is a security risk that allows user login credentials to be stolen.

I also see indications of warnings on other forms (like in developer tools the URL is red when doing a form and it's not using SSL).

Also, nearly all the big sites are forcing SSL connections for their sites.  I wouldn't be surprised if big sites like Facebook start also showing preference to posts linking to secure sites.

I think most websites have it backwards.  They pay to have privacy (decreasing a visitors trust) and they don't pay to use SSL.

In short, in my opinion if your site is worth more than a couple hundred dollars than SSL is worth it.