Alex,

Another approach would be to bind the SpinIncrement property to the Value property and use a converter to set the increment.  For example:

XAML:

<Grid>
    <Grid.Resources>
        <local:SpinIncrementConverter x:Key="SpinIncrementConverter"/>
    </Grid.Resources>
    <igEditors:XamNumericEditor SpinButtonDisplayMode="Always" SpinIncrement="{Binding RelativeSource={RelativeSource Self}, Path=Value, Mode=OneWay, Converter={StaticResource SpinIncrementConverter}}"></igEditors:XamNumericEditor>
</Grid>

C#:

public class SpinIncrementConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        double newValue = 0;
        int increment = 1;
        if (value is double)
        {
            newValue = (double)value;


            if (newValue >= 100)
                increment = 10;
            if (newValue >= 1000)
                increment = 1000;
        }
        return increment;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Let me know if you have any questions with this matter.