What is the reason for the high costs?

Unfortunately, I don't have a clear reason for the high costs.
As I mentioned earlier, I've closely monitored the CPU and RAM usage, and there haven't been any significant deviations from previous months. All usage patterns seem consistent with what I've observed in the past. Therefore, the sudden increase in costs remains unexplained.

I hope someone else can shed some light on it.

Please check what the reason for the costs is, if you weren't aware, you can view a cost breakdown from within the project's cost card.

You mean the Usage by Project, where I can see how much CPU and RAM my projects consumes? I already checked it, but I don't see anything that would justify any increase in costs. It is all like it has been before, aroung 500-1000mb of RAM usage and 0.1 vCPU usage with some spikes up to 1.4. Everything looks normal

But I am starting to assume that these are the costs of the past 3 months ALTOGETHER. Which are already paid off by my prepaid purchase. But how can I be sure?

You have access to your past invoices and all your current usage for any given project broken down by service, please look into that.

Thanks for your help. I have read your answer thoroughly and I looked into my current usage, which is displayed for the last 3 months. And I looked into the past invoices.
And I can confirm. There is nothing justifying the increase in current costs.
I can only notice, that the costs are not invoiced (yet). Only my prepaid credits are used.

So I am still wondering what these 30$ are about.

I am still wondering what these 30$ are about.

The user is able to look into these matters, you can view your cost breakdown per service for every billable resource type.

And I did exactly that. I looked into the graphs for CPU and RAM, there is nothing unusual, everything exactly like it has been the past months.
I looked into the invoices of the past month, which confirms my usage. And yet, my current costs say they are about 30$,

I can not look more into it, as I already checked the breakdown per service for every billable resource and the invoices.

I have a feeling you are looking at your service metrics, instead of the cost breakdown.

On the Usage Page at the bottom is a section called Usage by Project which you can open up to see the resources used in the past and Project Cost which display the costs per CPU usage and Memory usage and so forth.
There is also a button/link to view costs by service of a project.

These numbers there add up to the 34$, but that does not explain, why yesterday before the credits have run out, the current costs where about 7$ after almost one months, which the invoice I got today proofs, and now over one night, it adds up to 34$.

I am thinking that these 34$ have nothing to do with current costs

What specific resource type is responsible for the high costs?

Memory usage was always the one source that costs the most every month. Specifically the mongoDB service which used Memory the most.

Unfortunately Railway is not able to offer discounts on resources used.

I am not asking for a discount on resources used, I am asking why my usage increased 6-fold overnight without any discernible reason. I mean, if there had been a DDoS attack on my service, well, that would obviously be a reason, but I am pretty sure, that this would be clearly visible in the Usage by Projects graphs but there are no noticeable signs pointing to that.

I am pretty sure that this is a mistake on behalf of Railway, my credits ran out naturally and now that all my services are paused and not generating any (current) costs, Railway maybe wants to display the costs of the past month instead of the current costs (which should be 0 as I paid everything with prepaid credits) but hasn't changed the label or so.

Railway is not able to offer insights on why your services have used the resources that they have used.