What cost per use means
Cost per use is what one piece costs you every single time it goes back on a bed. Work it out like this:
cost per use = (price of the piece ÷ number of times it can be washed and put back into service) + what one wash costs you
Two sheets can carry exactly the same price on the invoice and be a long way apart on this number. A sheet that survives twice as many launderings costs you half as much per night, even if it was dearer to buy. A sheet that goes grey, tears at the hem or shrinks out of size after one term costs you far more than it saved.