# cost-per-decade
Buy-it-for-life arithmetic: cost per decade, cost per use, and the
year the cheap option stops being the bargain.
Sticker price is a bad comparator when lifespans differ. A $40 item
replaced every two years outruns a $280 item that lasts twenty: $140
per decade against $200, and the forty-dollar stack needs fifteen
units to cover thirty years.
The package follows the cost-per-decade method used on
[Durable Picks](https://www.durablepicks.com). It depends only on base.
## Example
A $280 durable item with a 20-year expected life, a $40 alternative
that lasts 2 years, a 25-year warranty, and 150 uses a year:
* cost per decade: `costPerDecade 280 20` = $140.00
* cost per use: `costPerUse 280 20 150` = $0.0933
* warranty: `warrantyClass 25 20` = covered for life (25y warranty on 20y expected life)
* 30-year horizon: durable = 2 purchases, $560 total; cheap = 15 purchases, $600 total
* break-even: `breakEvenYear 280 20 40 2 30` = year 12
The cheap stack hits $280 at year 12, the same year it takes its
seventh unit. From there the durable option saves $40 across the
thirty-year window, and the warranty outlives the item instead of
covering a fraction of it.