RetireJapan

When we're talking about the "average share cost" we're talking about the average price you've paid per share over the course of all your investments in that fund. So if you've made multiple investments the running tally would look something like this, but the only important number at the time of sale is the latest value in the Avg Share Cost column here.

(Sorry for three replies in a row)

And if you want the info straight from the horse's mouth then here's the NTA page on this calculation (though it's in Japanese).

https://www.nta.go.jp/taxes/shiraberu/t ... u/1466.htm