How to track currency exchange using ledger-cli?

I want to keep track of the full price I’ve payed for investments on ledger, to know the weighted average price.

So given the following register taked from the documentation:

2004/05/01 Stock purchase
    Assets:Broker                     50 AAPL @ $30.00
    Expenses:Broker:Commissions        $19.95
    Assets:Broker                  $-1,519.95  ; note_1

I can use an automated transaction to know how many AAPL I have using something like:

= expr ( commodity == 'AAPL' and /^Assets:/)
    (Allocation:Equities)             1.000

This way I can now how much AAPL I currently have, but I cannot know how much I have invested, specificaly I would like to keep track of all the postings like the one with the note_1 on the first example.

This way I can keep track of the total price I’ve payed for the investments I have, that can be calculated by:

$-1,519.95 / 50 AAPL

Is there any good way to keep track of that?