A couple · US$845 each
Atelier Classic at US$1,690 divided by two. The shared tour wins on cost here, and the desk will say so.

Rates & dates
A shared Nusa Penida day advertises at US$40–55 a head. Rebuilt from the bottom up it lands at US$130–180 — and there is a group size where private goes cheaper.
Below about six guests, the shared tour is genuinely cheaper. Above it, private is.
Hotel transfers
Conservation & park
Lunch
Snorkel equipment
Photography
Kelingking hike
Realistic per head
The private figure bundles fuel, crew, chef lunch, photographer, snorkel and SUP equipment, hotel transfers and all permits. Nothing is presented at the jetty that was not in the quote.
Per-person maths
The private headline divides by group size. These are the three numbers that decide it.
Atelier Classic at US$1,690 divided by two. The shared tour wins on cost here, and the desk will say so.
Still above the realistic shared number of US$130–180. Close enough that the experience gap starts deciding it.
Below the realistic shared number. At ten guests the private charter is the cheaper buy as well as the better one.
Send how many are coming and the date. If a shared tour is genuinely the better buy for your number, the desk will tell you that instead.