DAX Measure with Nested IF Statements

Dear community,

I would like to create a table per article where I calculate it's price, depending on the quantity that gets ordered.

To explain it in a easier way:

If the quantity that is ordered per bill is >= 10, I do fill the measure S1 with "1".

If the quantity that is ordered per bill is >= 20, I do fill the measure S2 with "1".

Accordingly the twp fields S1 and S2 have different discounts (e.g. 5% and 10%)

Net Price = if([S2]>=1 ;([Gross_Price]*(0.9));if([S1]>=1;([Gross_Price]*(0.95));
[Gross_price])

My measure works fine, when I do show in the table the article AND the bills (row by row).

I even managed to get the table total line correct with the following formula:

AVERAGEX(
 KEEPFILTERS(VALUES(Table[BILLINGS]));
 CALCULATE([Net_Price])
)

My problem starts if I want to show the data only per article (leave out the billing dimension).

My measure then does not calculate row by row on the level of the bills.

How can I adjust my Net_price measure with the if conditions so that it calculates on level of the bills?

Thanks for your help.

Nicole