Figuring price for a new line is easy most of the time. If you have a non unlimited plan, the price would be the same as the other lines.
Example: You have a plan that's $20 per line, a new line would be the same $20.
If you have a current unlimited plan, you go to the plan section on the website and set the line counter anywhere from 1 to 5+ to see what each line would cost. For older mix and match unlimited plans, that line would have to be one of the current offerings making finding cost the same.
A pin is just for authentication. Being an account manager means they can do everything you can as owner except changing the pin or adding managers. This includes disconnecting your number and ordering equipment without your say so. Don't add authorized users if you don't trust them.