Do you've have 1500 Everything Data Plan or 1500 Everything Messaging Plan? If you've 1500 Everything Data Plan then its going to be 19.99 plus taxes plus $10 data premium fee( if you're using Smartphone on the 3rd line).For 1500 Everything Messaging plan adding 3rd line is going to be only $ 9.99 plus taxes. You can always go to http://shop.sprint.com/mysprint/shop/plan/plan_wall.jsp?INTNAV=ATG:HElans&ECID=vanitylans to find more info on plans.Let me know if you have any questions on adding another line to the a/c.
Hope this helps
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