Who I have the same questioned this topic

Without going into unrelated detail, please explain the fee structure for otherwise identical plans in which

1. Existing customer purchases a replacement device from Verizon, vs.
2. Existing customer brings a replacement device purchased elsewhere

By "unrelated detail," note that I am not asking for possible differences related to

1. amortized cost of the device
2. trade-ins or rebates
3. the cost of different data plans, including shared plans
4. the cost of adding a new line

TIA