Estimate laptop lifespan.
A laptop's real cost is depreciation plus upkeep, not the price on the receipt. Spreading the purchase price across the expected useful life gives a monthly depreciation figure, and adding a twelfth of annual repair spend gives the true monthly cost of ownership. That single number is what makes a fourteen-hundred machine kept for five years directly comparable with a cheaper one replaced every two.
Monthly cost of ownership
Monthly cost = purchase price / expected life months + annual repair spend / 12
Residual value
Residual = purchase price x (1 - age months / expected life months)
Four to six years is typical before software support, battery degradation or performance make replacement sensible. Business-class machines with replaceable batteries and available parts commonly reach the upper end.
No — real resale value drops fastest in year one, often thirty to forty percent. Straight-line is a budgeting convention, useful for cost comparison rather than for predicting sale price.