The short answer
Add the required service, taxes, fees, trade-in value, and lost flexibility across the full credit period.
Step by step
Record the phone's retail price and the exact monthly credit amount and duration.
Add required plan costs, activation, taxes, fees, down payment, and any upgrade or insurance charges.
Subtract the honest market value of the trade-in you surrender.
Calculate what you owe and which credits disappear if you leave or change plans early.
Compare the total with an unlocked phone on the least expensive suitable plan over the same number of months.
Before you finish
Keep screenshots, receipts, chat transcripts, confirmation numbers, and the first bill after a change. Carrier terms and account eligibility can change, so verify the final price and conditions directly before submitting an order.