1. What does it mean?
넘사벽 shortens 넘을 수 없는 사차원의 벽, an exaggerated 'four-dimensional wall' you cannot get over. It describes someone or something so far ahead that comparison seems pointless.
Also written or said as 넘을 수 없는 사차원의 벽, neomsabyeok
2. The nuance
It is praise or comic resignation, not a precise ranking. You can use it about skill, scale, or difficulty; it does not have to mean expensive or physically tall.
3. Examples
“친구의 종이접기 실력은 나한테 넘사벽이야.”
My friend's origami skills are on a completely different level from mine.
“작은 공연장인 줄 알았는데 무대 규모가 넘사벽이네.”
I expected a small venue, but the scale of the stage is in another league.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.