This is simply the average portfolio cost of all trading days in the year so far.
Example: There are three trading days so far in year 2017 [2017-01-03, 2017-01-04, 2017-01-05] and the portfolio costs are (SGD15), (SGD15) and (SGD30) respectively.
Then the average cost would be (15 + 15 + 30) / 3 = SGD20.
