There's two different things you're talking about here. There's the cost estimate- the amount you tell people up front a surgery is likely to cost, vs the actual cost to the hospital. You don't need to guess how many syringes of X you're going to need when you've already done the surgery. At that point you just addd up all the stuff you used, price it up and that's the BOM. Cuban is saying they don't even do that - they don't track their costs on a surgery by surgey basis at all. And because they don't track any of that it's impossible to reason about it.

It's impossible to answer "How much will it actually cost to insure person X" because you don't have any of the data on what the costs will be when person X needs a given surgery, all you have is the aggregate costs of the entire system - a lot of which is misleading because things are cross-subsidized because no one is really tracking costs. It may well be that whilst every surgery is billed equally in reality obese patience are responsible for 80% of the cost. Or it may even be that the hospital is making an average loss on hip surgeries because their negotiations with the insurer drove those prices down whilst brain surgeries give a nice profit margin.

And so you can't ask "How much would it cost for the government to fund service X" because you don't know how much it costs, all you know is the aggregate money spent across all medecine.