I don’t know if they are the cheapest, but the Migros Print Shop in rue du Prince in downtown Geneva offers very good and helpful service – and if you have a Migros customer card you will earn “ points” on the cost.

And should you be likely to have more print jobs of the sort, it’s worth inquiring about their rates for regular/frequent customers.

Also, avoid going around 4:30 p.m. if you can because that’s when the “collège” students are there in force.

I would second Ricthie as have used Migros print shop, but at Painpalais Rue du Conseil-Général, 20.I ff neither of those are convenient they are also at Balexert ( which I used a few years ago), Cornavin, Lancy -Onex, and La Praille. All the addreess for thm can be found at https://www.migrosprintshop.ch

If you simply need black and white for you (and not for commercial like flyers or catalogs) better go to a library, it's the cheapest option.