Forget it. You won't have time for it. Most students barely have enough time to do their own laundry and cleaning, let alone anyone else's. And if you hire people to do the actual work, you'd have to organize their schedules, pay their wages, keep track of tax withholdings, etc.
But to answer your question, I used a laundry service sometimes in college (when things got really out of hand LOL). I remember they charged by the pound. Laundromats will often perform this service, so you can find out how much they charge (perhaps $1.50/pound? It would depend a bit on location).
I doubt anyone will pay to have their dorms cleaned. Those who care will do it themselves; those who don't will just not do it.