The concept of 'efficiency of scale' does not translate to service industries. The only efficiency that happens is efficiency of funneling more money to higher salaries of higher executives. Hospitals are driven by profit and their incentives do not align with the precepts that led to their formation.
By the way, actual cost transparency has winners and losers too. The winner is generally private insurers, who can use it to negotiate reimbursement further downward. The losers are everyone else. It's pretty much never the patient, regardless of what advocates of "patient consumerism" cry. When you're a grain of sand between two massive gears, you don't ever win the game of policy arbitrage.
If all you need is stitches, you can go to your PCP ($20-$60), surgeon's private office ($50-$100) or Urgent Care ($50-$200). Instead you go to the highest overhead center, occupy the attention of at least 2 nurses, a mid-level, and a doc, and... yes, pay for all of the above. In a space whose allocated overhead includes the weighted average of "stitches that shouldn't have come in" to "diabetic ketoacidosis with multiple organ failure."
Part of our systemic problems is, due to how we're structured, it's borderline impossible for an ER to say (a polite version of) "You're fucking kidding me. We have an urgent care center across the street - go there." [1]
We have many, many, many systemic problems. People using the ED as a primary care office and complaining about the disparity in prices is the least egregious of them.
[1] In part because hospitals used to try to bounce indigent patients. So now they can't do anything that smacks of bouncing anyone. So even if you try to divert patients from the ED to an in-house Urgent-ish Care, they still have to go through the ED pathway to determine that they're stable (meaning a doc has to evaluate them), before they can be shunted over. And now you're in a hospital, so the overhead of the Urgent-ish Care is already way higher than just having gone to UC to begin with. The hospital has no incentive to establish a spot for providing the same level of care, at high overhead, for lower reimbursement.