They're definitely exceptional cases, but in general tropical fruit (mangos, pineapples, even bananas) can be up to two or three times the price you might be used to paying in the US (or even Australia - not sure what it's like now but when I was in the US 15 years ago I remember bananas being especially cheap even in supermarkets - here you generally have to go to dedicated fruit/veg markets to get fresh produce cheaply).
Bananas are cheap in part because they are clones and incredibly predictable in terms of when they will ripen (therefore when to pick them, when to ship them, etc).
Yes but that doesn't explain why they would be substantially cheaper in US supermarkets vs Australian ones (after all, they're all grown here!), or why they're so much cheaper at dedicated produce markets. Apparently you can buy 8 bananas for $1.50 at a US Costco - not sure what Costco prices here are like (I do happen to live near one of the few stores in the country but have never been), but it wouldn't be unusual to pay almost $4 for that many at most supermarkets here, or around $2-3 (sometimes less) at a market, assuming 8 bananas weigh roughly 1kg. I can't find a reliable source for Japanese supermarket prices but my memory was something close to 400¥/kg a few years back, where that translated to ~$5 AUD. Mangoes could be 800¥ each, easily double or even triple the price they are here, and yes, watermelons (even the regular kind) are especially expensive, 1000+¥ for a smallish (~1kg) melon would be considered a bargain. Interestingly they're often grown locally (the super expensive square ones exclusively so), from what I can read, though I gather most are imported.
Bananas shouldn't be used as a baseline. USA supermarkets explicitly use them as a loss leader because they've discovered that nobody comes in to buy just bananas when they want bananas.
