What is the average medical cost of a sonogram/ultrasound?

I'm 23 weeks pregnant and very impatient. I really want to know the of my baby but don't have insurance. I'm seeing an OB/GYN regularly and he said he's not worried about the health of the baby and if I wanted to get an ultrasound, it would be only for my curiosity at this point. I will eventually need a diagnostic/medical ultrasound though. I'm planning on going to an ultrasound facility with trained professionals and a very good reputation which will cost $189 for 3d and 4d imaging. I'm just wondering if I would be better off to go to the hospital to have everything done at once or if it would be outrageously expensive due to having no insurance. What would be the average cost of a diagnostic ultrasound at a hospital or Dr.'s office?