score:5
Usually answers to questions like this are very complicated. In New Caledonia, though, life is blissfully simple!
There is exactly one mobile operator in the island, OPT. They're also the only fixed line phone operator, only ISP and only one postal service.
OPT has exactly one prepaid plan, LibertΓ©. The SIM costs 6,195 XPF, which includes 3,000 XPF of credit. All local calls 44.10 XPF/min, while SMS are 12/42 XPF to local/international numbers.
With this plan, you can also purchase Internet data in exactly... wait for it... two sizes: 90 F for one hour/100 MB (Mo = mΓ©ga-octet), social networks free, or 400 F for 24 hours/200 MB.
All this works out to US$60 for 3.3 GB, or around $20/GB. Gulp! Better stick to the hotel wifi.
Update: ...or so I thought, but turns out hotel wifi is universally (n=2) awful while OPT's mobile internet is pretty zippy and accessible virtually anywhere you'd want to use it. Also, the MB caps are soft so the connection continues to work over the data limit, only more slowly.