Railway's hobby plan costs 5 dollars a month, so you'd be spending a minimum of 5 dollars on Railway. How it works is you pay for hobby plan which costs 5 dollars, you then get access to Railway, and you pay for the resources you use as specified in Railway's Pricing Page. However, hobby plan includes 5 dollars of free usage a month as well.

Meaning, if you have an app that uses 3 dollars a month, then you'll pay 5 dollars a month for Railway + 3 dollars of usage, except the first 5 dollars of usage is free, so that means the 3 dollars of usage would be free, and therefor you only pay 5 dollars. On the other hand if you're hosting an app that uses 8 dollars a month on Railway, then you pay 5 dollars for hobby plan + 8 dollars of usage, but the first 5 dollars of the 8 dollars-usage is free, and therefor you only pay 3 dollars of the 8 dollars, making the total 8 dollars (5+3 = 8).

You can think of it as solely paying for the resources you use but the minimum is 5 dollars.

But if I only used 0.0003, do I need to pay 5$ every month?

Yes, using Railway costs 5 dollars a month, so you'll be paying 5 dollars a month minimum

Ok, thank you
Have a good day

No problem, you too :)