How do we calculate salvage value on printers?

It depends on the type (inkjet, laser, LED page printer, phaser/solid ink, dot matrix, line printer, daisywheel, etc.), age, amount of use/wear, physical condition, and more. The best advice is to speak with a professional printer repair shop to get an idea of what such a printer might be worth. If you have printers that you wish to get rid of for, then you may be able to sell some of them (such as laser printers) to the aforementioned repair shops for parts. Inkjet printers are typically designed to be throw-away items, so if they are not working, they have no practical value.