>You could do this in XAML using a value converter:

  public class BalanceConverter : IMultiValueConverter
>  {
>    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
>    {
>      var valuesToConvert = values.Cast<string>().ToArray();
>
>      double val1;
>      double val2;
>      if (!Double.TryParse(valuesToConvert[0], out val1))
>        val1 = 0;
>      if (!Double.TryParse(valuesToConvert[1], out val2))
>        val2 = 0;
>
>      return String.Format("Balance: {0}", (val1 - val2).ToString("C"));
>    }
>
>    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
>    {
>      throw new NotImplementedException();
>    }
>  }

<Window x:Class="WpfApplication3.MainWindow"
>        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
>        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>        Title="MainWindow" Height="350" Width="525"
>        xmlns:local="clr-namespace:WpfApplication3">
>  <Window.Resources>
>    <local:BalanceConverter x:Key="balance"/>
>  </Window.Resources>
>  <StackPanel>
>    <TextBox Name="txtDepositTotal" />
>    <TextBox   Name="txtDep_Amount" VerticalAlignment="Center" />
>    <TextBox  Name="txtBalanceOutstanding" >
>      <MultiBinding Converter="{StaticResource balance}">
>        <Binding ElementName="txtDepositTotal" Path="Text"></Binding>
>        <Binding ElementName="txtDep_Amount" Path="Text"></Binding>
>      </MultiBinding>
>    </TextBox>
>  </StackPanel>
></Window>

(Converter logic could be tidier :-} )

Viv! This looks like the bees knees! Thanks very much.

Would you recommend doing something like this to display the sum of row values from a grid? (For example in this datagrid to show the DepositTotal.) Or is what I am currently doing the better way? I currently call some code in th egrid's celledit_ending to sum the crt_Amount value and store it in the DepositTotal variable which is bound to the txtDepositTotal textbox.