You can tell how much it will cost to add a team member by going to your account settings and clicking into the "Team" tab. To get there, click your name at the top right of your trips page, then account, then Team.
You can get a quote for what it will cost you to add a new team member by entering the name and email of the teammate you're considering adding. The right side of the screen will update what your billing terms will be. You'll need to actually "Confirm Changes" to have that go through so if you're just looking at what it will cost you, don't click Confirm.
The prorated amount will be determined by taking the amount of time remaining until your renewal date and use that to calculate what you'll be charged.
Price = (Remaining Days Until Renewal / Total Days In Your Subscription) * Base Rate * Discount
Discount info about adding new team members can be found here: Volume Pricing For Teams and Multiple Agents
If you have another discount applied to your account such as an agency rate, the greater of the two discounts will be applied. They do not stack.