In article ,
Bonobo wrote:

1. I am really gratefull for the help, it works!
2. I am really frustrated as I don't understand the formula...

Let's assume that A1:C6 contains your data, and that the data is
filtered for 'x' with the following result...

Row 1 Label1 Label3 Label2
Row 2 x red 10
Row 4 x red 12
Row 6 x blue 15

If we have the following formula...

=SUMPRODUCT(SUBTOTAL(3,OFFSET(B2:B6,ROW(B2:B6)-MIN(ROW(B2:B6)),0,1)),--(B
2:B6="Red"),C2:C6)

SUBTOTAL(3,OFFSET(B2:B6,ROW(B2:B6)-MIN(ROW(B2:B6)),0,1)) evaluates to:

{1;0;1;0;1}

Visible cells containing data are assigned 1 and hidden cells are
assigned 0.

--(B2:B6="Red") evaluates to:

{1;0;1;1;0}

Each cell is evaluated as TRUE or FALSE, which is then coerced by the
double negative '--' into its numerical equivalent of 1 and 0,
respectively.

C2:C6 evaluates to:

{10;16;12;18;15}

SUMPRODUCT then multiplies the evaluations...

{10;0;12;0;0}

....which it sums, and returns 22.

Hope this helps!