Calculate energy costs for a sensor

Hi,
I want to have a counter, that is calculating the monthly energy costs for a specific sensor.

I have two Nous Powerswitches, that I combine into sensor.stromverbrauch_ngt
That Helper have a cumulated kWh value, with a always increasing curve.

I have as well the sensor.hackelbrink_1_strompreis with Euro / kWh. That one is changing on a hourly base.

My Idea to create a derivate failed

sensor
  - platform: derivative
    source: sensor.stromverbrauch_ngt
    name: "Stündlicher Verbrauch Gewerbe"
    unit_time: h
    round: 2

In that sensor, I have no cumulated view anymore.

Where is my mistake?
How can I get to my monthly energy consumption for that sensor?

Thanks

The derivate looks like this
image

Use a Helper