Write a 16K cost (16.000) on the Cost field. Now F2 to edit, and change sign. The value becomes 16, as the dot is misinterpreted.
Where to define that in my case? I use dot as thousands separator
Numeric format is locale-dependent, but apparently the locales are messed up. How do you type the initial value? Just “16000”?
Yes. And press ENTER. 
Shows my local 16.000 (thousands)