You don't know what size the kidney stone is. It could be too big to pass or it could be lodged in your ureter. If it were going to pass, I really think it would have done it by now. I don't have insurance either, so I avoid the ER as much as possible.
I don't think they'll be able to do much for you in the ER besides give you some pain meds and refer you to a urologist.
A regular doctor can do the same thing less expensively, if you can wait to get in. But if the pain is too much or you start to show signs of infection, then the ER is the place to go.