Compare laptop purchase, financing and upgrade options.
Trading in an old machine reduces the amount to be financed, which is why it is applied before interest is calculated. The monthly payment uses the standard amortising loan formula, so the total paid exceeds the price by the interest shown. Comparing that interest against the cash price is the decision: at a low promotional APR financing is cheap, and at a retail APR it can add a tenth of the machine's price.
Financed laptop cost
Monthly payment = principal x monthly rate / (1 - (1 + monthly rate)^-months), where principal = cash price - trade-in
Only if the cash price is unchanged. Where a discount is available for paying outright, that forgone discount is the real interest cost.
Private sales usually fetch more but take time and carry risk. Enter whichever figure you will actually realise.
If the interest shown is small relative to the value of having the machine now, it can be reasonable. At double-digit APRs over two years, the interest becomes a meaningful share of the price.