Else bits_per_month influence the weight of the monthly fixed cost

I'd state the exact other way round: Scaling waiting times will influence the ratio of the monthly fixed costs!

Let's take a fictional, very simple example:

We have a fictional vehicle that waits for exactly a one month in a bpm=18 setting.

Let's increase bpm to bpm=22 and scale the waiting time:

It will still wait a whole month but that waiting time will cost much more!

Let's have a look at an example schedule:

A: 1 minute loading

A->B 8 minutes journey time

B: 1 minute loading

In a fictional simuworld at 10 Minutes per month this makes exactly one cycle per month.

Let's increase month length by a factor of 8:

There will be exactly 8 cycles per month

That means 8 times the total waiting time and 8 times the total journey time (and distance)

That means 8 times the revenues (given constant demand) and 8 times the costs.

Seems correct to me.