The idea comes from Your Money or Your Life, by Vicki Robin and Joe Dominguez: money is life energy exchanged for hours. Divide your take-home pay by the hours you genuinely give to work, including commuting, training and the time it takes to switch off, and you get your real hourly rate.
With that figure, a $900 phone stops costing $900 and starts costing, say, seventy-five hours. Almost two weeks of work. Neither number is truer than the other, but only one is in the unit you actually pay in.
It is not a technique for spending less but for spending with the full information. Plenty of purchases still make sense expressed in hours; some stop making sense immediately, and that is precisely what the exercise was for.