Ha! Cost nothing? That's rich! I mean, sure, it doesn't cost anything to tell people they must wear a mask, but enforcement costs. Lack of compliance costs. Purchasing and maintaining masks cost. Taking the time to wear them and change them out properly costs.
That is actually a step too far for the enforcers, not even they care enough to make sure people are doing it properly. Or that employers are providing time and enough masks for employees to use masks properly, defeating any point mask mandates might have ever had if masks were effective against virus spread to begin with.
Cleaning up filthy masks that have been carelessly discarded costs. Carrying out manual labor with decreased blood oxygen levels costs. Kids, who aren't the greatest with hygiene, being forced to wear masks and not being given guidance on wearing and changing them out properly who end up with staph infections on their faces cost.
That's just the stuff off the top of my head.
I guess you could say mask mandates don't cost anything, if truth isn't important.