Japanese animation blu-ray are often, as strange as that may sound, even more overpriced:
After a quick look, Perdona5 seems to have at least 10 boxes. Not ridiculous since they tend to contain 2-3 episodes. But each is 6600 JPY. That's more than 600 dollars. Even in VOD, I've seen some website that make you pay 150 JpY per episode... And 4000JPY for the last one.
The US prices are high compared to France ones (we can get 100 episodes of Monogatari Séries for 90€, but that is a discount compared to the japanese price)