That calculation is definitely an important consideration but "reparability" does not have to mean "keep an outdated machine running indefinitely".
Reparability includes design that enables troubleshooting.
For example: Run the "$x to fix versus $x+$y" on a non-functional fridge. A fridge designed for repair might have a thumb-screw panel for access to the logic board with clearly labeled test pads for fuse continuity. This (or other, similar repair-friendly designs) would allow you to more easily determine the value of "x" and more accurately assess whether "x" is small enough to make repair the more viable option.
Reparability includes design that enables troubleshooting.
For example: Run the "$x to fix versus $x+$y" on a non-functional fridge. A fridge designed for repair might have a thumb-screw panel for access to the logic board with clearly labeled test pads for fuse continuity. This (or other, similar repair-friendly designs) would allow you to more easily determine the value of "x" and more accurately assess whether "x" is small enough to make repair the more viable option.