The Trade-In Math: What the Deal Is Actually Worth
Most people evaluate a trade-in promo by looking at the headline credit and the price of a used handset, subtracting one from the other, and feeling clever.
That subtraction leaves out four terms, and two of them are usually larger than the ones people remember.
The formula
Net gain = promo credit actually received
− price of the trade-in device (incl. tax and shipping)
− plan cost increase × months required
− any new-line cost × months required
− value of what you gave up (your old phone's private-sale price)Every term matters. Working through them in order:
Promo credit actually received is not the headline. It’s the headline multiplied by the fraction of the credit period you’ll actually complete. Planning to switch carriers in a year against a 36-month schedule means you receive roughly a third.
Device price including tax and shipping. On eBay, tax and shipping commonly add 10–20% to a low-value handset. On a $60 phone that’s real relative to the price paid.
Plan cost increase × months. The term people forget, and frequently the biggest one. A $15/month increase across 36 months is $540 — enough to erase most of a mid-sized promo on its own.
New-line cost × months. If the offer requires adding a line, this is a large recurring cost, and unlike the plan delta it doesn’t end when the promo does unless you cancel the line.
What you gave up. If you traded in your own good phone rather than a cheap sourced one, its private-sale value is a real cost. This is precisely the term the sourcing strategy is designed to shrink: buy a $70 eligible handset, trade that, and sell your own phone separately.
Worked example one: the deal works
All numbers below are hypothetical. Substitute yours.
A carrier offers $800 off a new flagship with any eligible trade-in, as 36 monthly bill credits, requiring their top unlimited plan. You’re on a plan $10/month cheaper. You’d have upgraded anyway and you’ve been with this carrier for years.
You buy an eligible three-generation-old handset on eBay for $65 plus $8 tax and shipping.
| Term | Amount |
|---|---|
| Promo credit (full 36 months) | +$800 |
| Trade-in device, delivered | −$73 |
| Plan increase, $10 × 36 | −$360 |
| New line | −$0 |
| Own phone given up | −$0 (sold separately) |
| Net | +$367 |
Positive, and meaningfully so — but note that the plan increase consumed 45% of the headline. The naive calculation ($800 − $73 = $727) overstated the result by roughly double.
Worked example two: the deal loses
Same hypothetical $800 credit, 36 months, but this offer requires adding a line at $25/month and moving to a plan $20/month more expensive. You’re fairly likely to switch carriers when your current promotional pricing ends in about 18 months.
| Term | Amount |
|---|---|
| Promo credit (18 of 36 months received) | +$400 |
| Trade-in device, delivered | −$73 |
| Plan increase, $20 × 18 | −$360 |
| New line, $25 × 18 | −$450 |
| Net | −$483 |
Same headline number. Nearly a $900 swing, driven entirely by terms that don’t appear in the advertisement — and the “up to $1,000” framing is doing none of the work here.
The lesson isn’t that big promos are traps. It’s that the headline carries almost no information about the outcome.
The four questions that settle it
Before comparing any offers, answer these:
1. How long will you genuinely stay? Be honest rather than optimistic. If you’ve switched carriers twice in five years, don’t model 36 months. Multiply the headline by your realistic fraction and use that number everywhere.
2. What’s the monthly delta, all in? Required plan minus current plan, plus any new line, plus any add-on the promo requires. Multiply by the months you’ll actually stay.
3. Would you have upgraded anyway? If yes, the new phone’s cost is not attributable to the promo — you were spending it regardless. If no, you’re being sold a phone you didn’t want with a discount as the argument, and the full financed cost belongs in the sum.
4. What’s the downgrade risk? If the device fails the condition floor it’s revalued at standard trade-in, which may be near zero, and you keep the full financed phone. Multiply that downside by your honest estimate of the chance and add it as a cost. Buying a device comfortably above the floor rather than exactly at it is cheap insurance.
Bill credits versus cash
One more adjustment, smaller but worth naming. Receiving $800 spread over 36 months is worth less than $800 today. At ordinary interest rates the discount on a three-year drip is modest — single-digit percent — so it rarely flips a decision.
It matters more as a risk discount than a time-value one. Money arriving monthly, contingent on continued good standing, on terms the carrier can adjust, is not equivalent to money in your account. Applying a haircut to the headline for that alone is reasonable.
Where to stop
Two shortcuts save most of the work:
If the monthly delta × months exceeds the credit, stop. No amount of clever sourcing recovers it. This kills more offers than anything else and takes ten seconds to check.
If you can’t complete the credit period, divide first and evaluate second. A 36-month promo evaluated at 12 months is a different, much smaller offer, and should be compared against a shorter-term one on that basis.
Both of those checks happen before you look at a single listing. Once the offer survives them, sourcing becomes worth your time — start with what to buy and what to avoid, then verify the IMEI before any money moves.