Which cost will be most difficult for Acme's insurer and Mark to negotiate following his injury?

The most challenging cost for Acme's insurer and Mark to negotiate is the mental anguish associated with never fully recovering. This difficulty arises from the subjective nature of emotional and psychological distress, which can be hard to quantify in financial terms. Unlike physical damages or replacement costs, which have clear market values and can be assessed based on receipts or invoices, mental anguish requires a more complex evaluation of the emotional impact on the individual. Factors such as duration, severity, and personal circumstances must be taken into account, making it significantly less straightforward to establish a monetary figure that both parties would find acceptable.

In contrast, the costs to replace tangible items such as shelves or windows and medical bills from surgery involve specific calculations based on existing prices and estimates, which can be easily substantiated and supported by documentation. These costs have a clear basis for negotiation as they involve factual estimates rather than subjective evaluations. Therefore, the intangible nature of mental anguish makes it the most intricate and contentious issue between the insurer and Mark.