Let's say the weapon purchase you're making is a big shoota, for the sake of example.
The codex entry (p. 100) states (emphasis mine) "For every 10 Orks in the mob, one Ork may
exchange his slugga or shoota for[...]"; thus, it is a replacement of his current weapon.
I'd list that as "10 Boyz, (slugga/shoota), 1x big shoota, 65 pts." - note that the exact style of listing varies from person to person.
For the Nob, the entry states "One Boy may be
upgraded to Nob", so the Nob replaces the Boy (and you have 10 models in total).
That would be "10 Boyz w/ Nob, 65 pts."
Let's say you're giving the Nob that big shoota from earlier - "10 Boyz, Nob w/ big shoota, 70 pts."
...or suppose you give him a power klaw, and give the shoota to someone else: "10 Boyz, 1x big shoota, Nob w/
PK - 95 pts."
Hope this helps