Okay, my biggest question is where the heck you've found these barns in the city of chicago?? It's a minimum 45 minute drive before I can even catch the scent of manure.
Along those lines, I'm not sure this is a DIY city. Given the time to commute, to either work or the horse, I'm not sure how many can really be relied upon to clean and feed the horses. Days seem to run particularly short here. If that doesn't happen you will find yourself spending a great deal of time managing people and dealing with complaints & accusations about who's not pulling their weight.
It's the dead of winter, yesterday's weather was lethally cold, and the sun is setting around 4:30. The barn will need lights and I'm guessing it's not heated. That's going to make it pretty miserable until about April and again I would have a hard time relying on a coop of strangers to make sure my horse was properly cared for.
I did coops in Santa Barbara where most of us lived within a mile of the barn. It worked because I could run and feed in the morning, come back around 10 to remove blanket, and the horses were mostly in paddocks that could survive a day or two if they didn't get picked. We were all comfortable with each other and could call and request help if we couldn't be around for farrier/vet visits, or if horse needed 3 doses of something each day, or if we noticed something just didn't "look" right. Even with all of this back-up it was still important (to me) that someone lived on the property fulltime, in the event of fire or some other horribly disasatrous event.
Are there crossties, grooming stalls, secure feed rooms, wash racks? What is the condition of the indoor and turnouts? Are there trails nearby, for the recreational rider you are likely to attract? Yes, in the budget you are speaking of the condition of the footing may not be a big concern, but I also would fear the price is so low you may attract the less than conscientious. How will you handle the horse whose owner drops him off and doesn't return for another month? or ever?
I am definitely curious because I'm currently priced out of horse ownership thanks to board prices that equal that of a studio apartment downtown, but at first glance I think this would be more of a hassle than a money-maker for you. I would raise those rates across the board, and then consider if you really want to be a "super" for a group of crazy horse people