My first thought is that I'd assume someone would also need another job (as you note) - because while housing would be taken care of, $500 for other expenses each month would not be enough in a majority of people's situations (paying for schooling, car, student loans, etc - the list goes on).
So do you have a hard time finding someone who would be able to have a second job with the hours you're requesting?
This arrangement could be majorly win win if you find the right person, but that may be tough. I'd advertise at the large university across the river from you.