Good money, if you maintain your equipment. Not only does a broke down washing machine or dryer cost you money, it inspires your customers to vandalize your stuff. Which costs you more money. My friend, and barber, up here owns one and it is a gold mine.
Same thing with those self-serve car washes. When I looked into those ten years ago, they cost about 350,000 to build, plus the land, and the 'poorest' one I could find was making something like $11,000/month. That was in San Bernardino and Riverside Counties.