Going from Sennheiser HD 650 --> Sennheiser HD 800S --> Focal Utopia, there are big differences to be heard.
I'd say the sweet spot is around $1500 (Sennheiser HD 800S) based on what I am hearing. For closed headphones, it is around $999 (AFC 2).
However for most people (friends & family), I recommened just buying the $300 headphone (Sennheiser HD 650) as they will never spend $1500 on a headphone and probably will not even appreciate what a headphone like the HD 800S brings to the table. Let alone the Focal Utopia with high-end cables.
What is actually funny, out of all my headphones, my girlfriend always steals the AKG 240 if she needs headphones. I would have thought she would try to steal the high-end ones
That also confirms to me that it is better to just recommend more affordable headphones to most people.