I had noticed how expensive square watermellon is and apparently the reason for that is because often times farmers will only grow one watermellon per vine as opposed to the normal four or five per vine because the logic is that by cutting off the other growing watermellons, you are saving the nutrients for the one watermellon thus making it taste much better and whatnot. Is this true? And if so, is it really that helpful to do so, because I would imagine that if a plant weren't able to handle that many fruits growing at the same time, they simply wouldn't grow at all, so simply cuting some off doesn't make all that much a difference?
I suspect that MOST of the cost is that this is a hassle/extra-labor item for the farmer (buying special boxes to put watermelons into, putting the boxes on the melons, etc.), rather than anything having to do with number of fruits per plant. It's a niche product and they will try to recover those costs as much as possible.
Thinning/reducing the number of fruits per plant does normally alter the fruit quality in a direction that suits people more than the plant. The plant wants to make as many seeds as possible, with only enough fruit/sugar/resources to encourage something to eat the fruit and distribute the seeds. The beneficial effect of thinning does, however, have limits.