Concatenate String with Bindings XAML

I'm thinking the .NET way of doing what you want, which is format value string as currency, is using the binding property StringFormat along with the Currency Format Specifier:

Text="{Binding totalCost, StringFormat=\{0:C\}}"

Your code would look like this

<Label Text="{Binding totalCost, StringFormat=\{0:C\}}"
   x:Name = "totalCost"
   HorizontalOptions = "Start"
   VerticalOptions = "Start"
   Grid.Row = "6"
   Grid.Column = "1"/>

Cheers.

See if this works for you:

Text="{Binding totalCost, StringFormat=${0}}"

Although the selected answer is correct, you will be ignoring commas in your currency values or the potential period. A better way would be to expose a get property that provides the appropriate value for the binding. Below is a code snippet for anyone in the future

<Label Text = "${Binding TotalCostFormatted}"
      x:Name = "totalCost"
      HorizontalOptions = "Start"
      VerticalOptions = "Start"
      Grid.Row = "6" Grid.Column = "1"/>

In your ViewModel

double totalCost;
public double TotalCost 
{
    get { return totalCost; }
    set 
    {
        totalCost = value;
        OnPropertyChanged(nameof(TotalCostFormatted));
    }
}
public string TotalCostFormatted
{
    get { return TotalCost.ToString("C0"); } 
}

Remember you can always just create a get property in your ViewModel that exposes some data. Call OnPropertyChanged or whatever your method is called that implements your INotifyPropertyChanged interface. If you want the cents on your dollar value, change "C0" to just "C".

For labels there is additional feature: ContentStringFormat, example bellow:

<Label Content="{Binding Tag, FallbackValue=Custom}" ContentStringFormat="Length: {0}" DataContext="{Binding ElementName=cbRebarLength, Path=SelectedItem}"/>