Pipo:
I found the problem.
> ...Win XP Pro french...
> I think I could be a problem with 
> country decimal because product 
> price is changing aswell when I 
> change the product weight.
     I replicated the problem changing the regional settings of Windows to French (it will also fail with any other country with different decimal separator).
> If I try to change the product weight 
> from 0,3 to 0,5 it saves 5 and the 
> product change from 15,50 to 1550 !
Yes. My cleaning routines try to strip non-numerical characters, deleting the ",".
I thought I was programming globally enough...   
 
Thanks for the bug report! I'm fixing this right now. 
Regards,
Mario A. Valdez-Ramirez.