This is not an easy answer and would require more information from you.
Do you need to purchase a phone? Would it be done on payments?
What plan do you have? How many lines are on your account?
I have one line on my account and I have Play More Unlimited. I want to add a line, also with Play More Unlimited. I want to purchase an iPhone 11 with 128 GB of storage.
Before adding the new line, this person would have been paying $80/month + taxes if receiving the autopay discount. After adding the line, this person will be paying $140/month + taxes if receiving the autopay discount PLUS $31.24/month for the new phone. The added cost per month is $91.24/month. If this person did not purchase a new phone on payments, the added cost would be $60/month.
This same person who had 3 lines of service and wanted to add another line would have been paying $165/month + taxes for service and after adding a line would be paying $180/month + taxes for service. This is only an increase of $15/month to add the line. With the phone purchase it would bring it up to $46.24/month of an increase.
So, as you can see, there is no easy answer to your question as I have shown you it can be anywhere from $15/month increase to $91/month increase. I could find ways it could even be lower or higher.
More information would be needed to see how much YOU would be paying in addition to your current bill.
Knowing all the pricing details when adding a new line is crucial. We would need more details to be able to answer this question. A private message has been sent to assist you further.