Then you have your answer. You need 500 bikes at $7.50 per day just to pay the rent. (based upon 20 business days a month). Therefore, since 3 bucks a day is too much for some, you would need a location central and close enough to thousands of commuters to draw in the needed 500.

It would seem more likely to find someone with an unused closet / office /store room that would store 30 bikes for five bucks a day, netting them an extra 3K a month for little work. But then again, who has extra unused closet / office / storage these days?

The old Chicago solution was when an economically disadvantaged kid would stand on the block and offer to "watch" your bike or car for a few bucks. When you came back for your car or bike, if it was still there and un-molested, the kid got his money. In the rougher parts there would be plenty of street parking, the kids rate was less than one third the fee for supposedly secure parking across the street.