Metal is purchased every working stock market day between 10:00-11:00 am (CET, Central European Time) at the current price of the selected savings product. The amount of the chosen product (accurate to ten thousandths of a gram) is purchased for the entire amount you sent the previous day.
So, if you save into a 1g bar, whose current price is 70 EUR, and the amount sent was 100 EUR, you will buy 1.4286 of this bar.
Detailed information about purchases can be found
here