In episode 20 of Hemispheric Views Andrew talks about his asset lifespan spreadsheet which shows the cost of an item, the date purchased, and the cost per week for said item.

I figured I'd have a crack at making my own and after some wrangling with formula in Numbers I was able to get something up and running. The key part was using the DATEDIF function to get how many days it has been since purchase:

=(cost_of_item / DATEDIF(date_purchased, todays_date, diff_type))

=(B3 / DATEDIF(B2, TODAY, "D") / 7) // for cost per week
=(B3 / DATEDIF(B2, TODAY, "M")) // for cost per month

Which gives me an output like this:

I don't know what I'm going to do with this information beyond obsess over it but I do know I'm getting my money's worth with those Audio-Technica headphones.