Thanx a lot James

I can't enter item cost unless 1$ or 10$ or 120$ impossible to add 0,35$ 1,76$

As you said the problem is from comma separate, 

I tried many times to enter the same item with the cost price, and close the QBook,& open it again

doesn't change anything. 

Checked left Menu QBook, edit, preference, Items & inventory, Company preference, automatic price & cost updates. 

just nothing changed.

Thanks 

AMO