public partial class Form1 : Form
{
    DataTable table;
    public Form1()
    {
        InitializeComponent();
        table = new DataTable();
        table.Columns.Add("Name", typeof(string));
        table.Columns.Add("Value", typeof(double));
        table.Rows.Add("John", 123);
        table.Rows.Add("Adam", 456);
        table.Rows.Add("Peter", 789);
        this.radGridView1.DataSource = table;
        var calculatorColumn = new GridViewCalculatorColumn();
        calculatorColumn.HeaderText = "Calculator Column";
        calculatorColumn.FieldName = "Value";
        this.radGridView1.Columns.Add(calculatorColumn);
        this.radGridView1.EditorRequired += RadGridView1_EditorRequired;
    }

    private void RadGridView1_EditorRequired(object sender, EditorRequiredEventArgs e)
    {
        if (e.EditorType.Name == "RadCalculatorEditor")
        {
            e.Editor = new CustomCalculatorEditor();
        }
    }
}
public class CustomCalculatorEditor : RadCalculatorEditor
{
    public override void BeginEdit()
    {
        var val = this.Value;
        base.BeginEdit();
        this.Value = val;
    }
}