I often get asked this question:

Costs:

Option 1

Server 1: Web server front end
Server 2: Standard back end server- SQL (you have this already- GP runs on this)

External connector unlimited non employees $2,019

Option 2

Server 1: Web server front end
Server 2: Standard back end server- SQL (you have this already- GP runs on this)

Windows CALs for each user/device, not SQL $30
These are not concurrent licenses

Note: When you install the server, you need to specify what kind of server it is- Web and the license type. CAL or per server