Thanks very much to all of you guys for your effort!!

The last code by Kenneth works best for me; I get the results the way I need them - in the column A the name of the .csv file, in the column B is the calculation, the only problem is, that in the column B all results are zero, thuscI suppose there is an error in the calculation part of the VBA code.

Time is not an issue, I have these 50k files (not 500k) divided into 500 subfolders, thus I can run the code in the particular subfolder and it is done in a sec.
I can repeat the code in all subdolders by myself, but if someone could even help me how to run the code through all the subfolders, I will be even more grateful

SamT: each source .csv file has max 200 rows

I really cherish your contribution, it's a huge help for me as I am in a time press to finish the diploma thesis, so I though that I could donate some smaller amount of my student money to your forum as a reward, if it is possible? For your time and effort.