The middleman demands to be paid.It would have to cost far more than any other country, because our health care already carries these higher prices. If implementing single payer caused our health care to cost similar to other nations, it would have to be because our new system makes Draconian reductions to benefit levels (which it couldn't realistically do or everyone would reject it).
Cost is only one important consideration though. There are numerous others.
If a hospital can offer me a 50% discount if paid upfront, then there is some serious wiggle home when it comes to actual cost.