I'm about to buy (火災保険）fire insurance right now too- it is a requirement of our loan. We've looked at Kakaku.com but I'm a bit suspicious of some of the very cheap insurers on there. I expect the devil is in the details but I have neither the time nor the advanced Japanese skills to go into those details right now. I did a mock online application through Rakuten and the total was not much different from the one described below.
We are probably going to go with AIG (through Aruhi loan company). We get a 28% discount, 18% because our loan is with Aruhi and a further 10% for applying online, plus some smaller discounts for meeting earthquake resistance standards etc. With the discount the total cost is around Y270 000 for 10 years of insurance for the house structure (including fire, flooding , typhoon etc but not house contents) paying out a maximum of Y13 million and 5 years of earthquake insurance paying out a max of Y6.5 million (half of the above). Insurance for flooding/ landslides is expensive but we had very heavy rain and flooding here a year or so back and the house is at the base of a (small) hill so we decided it would be wise to include that. As for earthquake Insurance, I was wavering but seeing as it's not all that much extra, we decided to be safe rather than sorry. Cheaper policies are probably available but it seems reasonable to me overall.
Would be interested to hear what others think.