Use the LabelFormat
<charts:ColumnSeries>
<charts:ColumnSeries.DataLabelSettings>
<charts:CartesianDataLabelSettings LabelPlacement="Outer" >
<charts:CartesianDataLabelSettings.LabelStyle>
<charts:ChartDataLabelStyle FontSize="10" TextColor="{AppThemeBinding Light=Black, Dark=White}" LabelFormat="€ 0.00" />
</charts:CartesianDataLabelSettings.LabelStyle>
</charts:CartesianDataLabelSettings>
</charts:ColumnSeries.DataLabelSettings>
</charts:ColumnSeries>