Just doing a little research, an Isami F-700 (Kyokushin's favorite) with one stripe (shodan) is about $65 USD. Add $4 per kanji character, and there's a lot of those, let's say 20 characters between the organization name and the karateka name, and you're talking $145 USD. Taxes, duties, shipping, exchange rates, etc. all thrown in there, and the $300 doesn't seem so astronomical. Excessive, yes, but not completely absurd.
I've seen videos of their belts being made. Isami hand embroiders, and embroiders before the belt is all sewn together. The belt is about as handmade as it gets with today's technology. And they're made in Japan.
I think it's hard to justify the cost of Kyokushin black belts. My reason why is that they get a new one at every dan promotion. At the higher dan ranks it obviously takes longer, but shodan to nidan is typically 2 years minimum, and nidan to sandan is typically 3 years minimum. If I'm spending $300 for a black belt, I want to wear it at every dan grade; just add a stripe. One of the reasons why I love Seido so much - you get promoted to the next dan rank, and they take your belt and add a stripe instead of giving you a new belt. The old belts have character IMO.