public class GridCellValidationEventArgs :
    GridRowValidationEventArgs,
    IDataCellEventArgs

Public Class GridCellValidationEventArgs
    Inherits GridRowValidationEventArgs
    Implements IDataCellEventArgs

<dxg:GridControl x:Name="grid">
    <dxg:GridControl.Columns>
        <dxg:GridColumn FieldName="ProductName">
            <dxg:GridColumn.EditSettings>
                <dxe:TextEditSettings AllowNullInput="False" />
            </dxg:GridColumn.EditSettings>
        </dxg:GridColumn>
        <dxg:GridColumn FieldName="UnitPrice">
            <dxg:GridColumn.EditSettings>
                <dxe:SpinEditSettings DisplayFormat="c2" />
            </dxg:GridColumn.EditSettings>
        </dxg:GridColumn>
        <dxg:GridColumn FieldName="Discontinued" />
    </dxg:GridControl.Columns>
    <dxg:GridControl.View>
        <dxg:TableView AutoWidth="True"
                       ValidateCell="OnValidateCell" />
    </dxg:GridControl.View>
</dxg:GridControl>

void OnValidateCell(object sender, GridCellValidationEventArgs e) {
    if(e.Column.FieldName != nameof(Product.UnitPrice) || !((Product)e.Row).Discontinued)
        return;
    var cellValue = (double)e.CellValue;
    var discount = 100 - Convert.ToDouble(e.Value) / cellValue * 100;
    if(discount > 0 && discount <= 30)
        return;

    e.IsValid = false;
    e.ErrorType = DevExpress.XtraEditors.DXErrorProvider.ErrorType.Critical;
    e.ErrorContent = discount < 0
        ? $"The price cannot be greater than {cellValue}"
        : $"The discount cannot be greater than 30% ({cellValue * 0.7}). Please correct the price.";
}

Private Sub OnValidateCell(ByVal sender As Object, ByVal e As GridCellValidationEventArgs)
    If Not Equals(e.Column.FieldName, NameOf(Product.UnitPrice)) OrElse Not CType(e.Row, Product).Discontinued Then Return
    Dim cellValue = CDbl(e.CellValue)
    Dim discount = 100 - Convert.ToDouble(e.Value) / cellValue * 100
    If discount > 0 AndAlso discount <= 30 Then Return
    e.IsValid = False
    e.ErrorType = DevExpress.XtraEditors.DXErrorProvider.ErrorType.Critical
    e.ErrorContent = If(discount < 0, $"The price cannot be greater than {cellValue}", $"The discount cannot be greater than 30% ({cellValue * 0.7}). Please correct the price.")
End Sub