I am not a business man, I just write the code, so I will refer you to the official pages where you can find information of the price of each premium service that Sucuri is offering. Here is the pricing page for the Antivirus [1] and here is the pricing page for the Firewall [2].
Also, there may be a misconception in the purpose of this plugin, this is a complementary tool for the premium customers, some people use it for free because there are some tools that do not require to signup, which means that there is no premium plugin, the premium services are a completely different thing.
I invite everyone interested to know more about the premium Sucuri services to visit these links (see below) and remove the /signup string from the URL to go from the pricing page to the description page, or simply navigate through the main website [3].
[1] https://sucuri.net/website-antivirus/signup
[2] https://sucuri.net/website-firewall/signup
[3] https://sucuri.net/
