I import mine from Japan so, most expensive single one? Hmm, let me check. Are we counting single purchse, single box of discs?

Single disc with full discount:

Kokoro Connect Hito Random 2 (3 episodes on BD plus bonus CD and box for discs 1-2):

ココロコネクト ヒトランダム 下 (初回限定版) [Blu-ray] [Blu-ray]

￥ 6,164

That's 26% off...

This one was a little more because I ordered it late, wasn't sure there would be any left and there wasn't much discount, usually there is 26-28% discount at amazon but this was not even 10% discounted. As it turns out, there were still lots left so I could have got it for ￥4900

It is just two episodes so the per episode cost is high, over ￥ 3000 / episode...

Mysterious Girlfriend X 2 (2 episodes on BD plus bonus CD):

謎の彼女X 2(期間限定版) [Blu-ray] [Blu-ray]

￥ 6,311

Most expensive box, the Japanese boxed sets run ￥20000-50000. So next time you complain about $50-60 for a Sentai set, it could be much worse!