Okay, after a little research, the three combined Islands do only have one stadium that meets IRB requirements, which is in Fiji (30 000 seater, with a portion of that for standing)
All the rest include 'capacities' that count for standing terracing, which the IRB doesn't want.
So, if a country needs a minimum of 8-10 stadiums, and we just say they build stadiums of 20 000 capacity (which already exceeds the entire Tonga/Samoa populations)
A semi-decent stadium of that size would cost 20 million or so pounds in the UK, and that all construction materials would need to be imported (never mind technicians/engineers etc..), the cost of building a single stadium would rise to 30 million or so pounds. Now times that by 10, and you already have a figure bigger than that of the combined Tongan/Somoan economies, and equal to about half of the Fijian economy.
Considering it is costing the NZRU 40 million pounds or something to host the RWC, and the money that would be needed to build roads/transport systems/ airports on 7/8 different islands, and already the cost exceeds 1 billion pounds (Fiji+Samoa+Tonga=800 million or so pounds)
And considering the Aberdeen city council has a GDP larger than that of the islands, and won't even help Aberdeen FC to pay for a new 30 000 stadium, well you can just imagine the re-action(s) of Fijian/Samoan/Tongan finance minister(s) when asked for 1 billion pounds
If they would want to host a RWC, Sevens would be the way to go, as they can all (bar tonga) win it, and they would only have to expand Fiji's stadium for the venue