I Googled oklahoma dialysis and got a list of cities that have centers. I would contact one closest to your friends and see what the cost is.
I don't know if this guy can help you or not but he lives in OK and he works with dialysis clinics and may be able to direct you to a center near where you want. (I sent his email in a pm)I think everything is negotiable and if I was paying out of pocket with my cash, I'd try and keep it under $300 per treatment. That's probably more than they get on the average for each patient each treatment. I think Medicare composite rate now is probably between $170-230 per treatment. You'll probably have physician fees and hopefully you won't need any additional medications but they would be extra too.