Rocket Source reads a period as the decimal separator and a comma as the thousands separator. Use 1,234.56, not 1.234,56.
Change this
1.234,56To this
1,234.56..,, then close Preferences...,, then select OK.If changing the display does not update your values, add a temporary column next to Cost or MAP. For a value in A2, enter =NUMBERVALUE(A2, ",", "."). Fill the formula down, copy the new column, then paste values over the original column.
Check a few rows before uploading. This conversion is for values that currently use a comma as the decimal separator.
Microsoft's guide includes screenshots of the Windows separator settings: Change the character used to separate thousands or decimals.