There are several ways you can approach this problem. The way I would try to do it is to make sure teh column type is set to double an dthe Format is set teh way you want it.
this.gridDataBoundGrid1.Binder.InternalColumns[1].StyleInfo.CellValueType = typeof(double);
this.gridDataBoundGrid1.Binder.InternalColumns[1].StyleInfo.Format = "#,###.00";
This should allow you to enter and see teh values. And if your users types an invalid valid, you will get a message.
If you want to make it so you user cannot type a invalid value, I would handle the CurrentCellValidateString event and vaildate the string with each key stroke to see if make sure the entry is valid.
private void gridDataBoundGrid1_CurrentCellValidateString(object sender, GridCurrentCellValidateStringEventArgs e)
{
GridCurrentCell cc = this.gridDataBoundGrid1.CurrentCell;
if(cc.ColIndex == 2)
{
double d;
if(!double.TryParse(e.Text, System.Globalization.NumberStyles.Any, null, out d))
{
e.Cancel = true;
}
}
}