private BindingExpression GetTextBinding()
{
    return this.AssociatedObject.GetBindingExpression(TextBox.TextProperty);
}

var bindingExpression = GetTextBinding();
object dataContextItem = bindingExpression.DataItem;
PropertyPath relativePropertyPath = bindingExpression.ParentBinding.Path;

public static T GetValue<T>(this BindingExpression expression, object dataItem)
{
    if (expression == null || dataItem == null)
    {
        return default(T);
    }

    string bindingPath = expression.ParentBinding.Path.Path;
    string[] properties = bindingPath.Split('.');

    object currentObject = dataItem;
    Type currentType = null;

    for (int i = 0; i < pathParts.Length; i++)
    {
        currentType = currentObject.GetType();
        PropertyInfo property = currentType.GetProperty(properties[i]);
        if (property == null)
        {
            currentObject = null;
            break;
        }
        currentObject = property.GetValue(currentObject, null);
        if (currentObject == null)
        {
            break;
        }
    }

    return (T)currentObject;
}

public static class PropertyPathHelper
{
    public static object GetValue(object obj, string propertyPath)
    {
        Binding binding = new Binding(propertyPath);
        binding.Mode = BindingMode.OneTime;
        binding.Source = obj;
        BindingOperations.SetBinding(_dummy, Dummy.ValueProperty, binding);
        return _dummy.GetValue(Dummy.ValueProperty);
    }

    private static readonly Dummy _dummy = new Dummy();

    private class Dummy : DependencyObject
    {
        public static readonly DependencyProperty ValueProperty =
            DependencyProperty.Register("Value", typeof(object), typeof(Dummy), new UIPropertyMetadata(null));
    }
}