I would like to multiply values from two columns and then display a sum of them in wingrid summary. Lets say its invoice summary, so its a summary of all prices multiplied by amount.

Forst idea was sum( [amount] * [price] ), which showed me #VALUE! error.

What I finally came up is:
sum( product([amount],[price])) which according to my understanding should first multiply two values from one row and then generate sum of that. At least displays some values but i'm having problems figuring out where are they taken from, and why doesn't it work the way I would like to :)

Can anyone help me a bit?