restoring the os from wiped hard drives

I get a lot of older computer which I recycle however I have many which are in working order except that the hards drives have been wiped clean and therefore they don’t have oprating systems. I am disabled and would like to get these ones running and donate some as well as sell a few. how do I do this without having to purchase new copies of xp for each one. Can the operating systems be restored? please help since they would not sell for more then 50.00 or so and XP cost over $100.00 what can I do I hate to waste working computers as scrap.