Thanks for your post. The cost of adding a line varies dependant on the price plan it is placed on and cost of the device. If the line is being added onto the account on a single line plan, it would incur the monthly cost associated with the plan chosen. If it is being added to a share plan, it would be $9.99/month if it was on an Everything Messaging Share Plan or $19.99 for an Everything Data Share Plan. If the device added is a smartphone or PDA, it would have to be on either a single line data plan or the Everything Data Share Plan and would also have a $10 premium data charge monthly associated with it.
The costs of the phones vary and each line would have a $36 activation fee associated with it.
Additionally, the account the line would be put on has to be approved to add an additional line. It some instances there is a limit to how many lines can be on the account.