Calculate Commission of trade

Hi all, 

I want to calculate the commission of a trade, my account is in GBP base ccy. 

I have the following code but I get the following error

23/05/2016 21:05:00.215 | Crashed in OnBar with NotSupportedException: GetSymbol is temporarily not supported in backtesting

  private double CalculateCommissionPer1k()
        {
            double commissionGBP = 0;
            string baseCcy = Symbol.Code.Remove(3, 3);

            // The Commission parameter is the cost each way per million. Calculate the full commission (in and out) per 1k of the base currency.
            double baseCcyCommission = (30 * 2) / 1000;

            if (baseCcy == "EUR")
            {
                // Use EURGBP to convert the EUR commission value to GBP.            
                commissionGBP = baseCcyCommission * MarketData.GetSymbol("EURGBP").Bid;
            }
            else if (baseCcy == "USD")
            {
                // Use GBPUSD to convert the USD commission value to GBP.            
                commissionGBP = baseCcyCommission / MarketData.GetSymbol("GBPUSD").Bid;
            }
            else if (baseCcy == "GBP")
            {
                // The base currency is the same as the account currency. No conversion required.
                commissionGBP = baseCcyCommission;
            }
            else if (baseCcy == "CHF")
            {
                // Use GBPCHF to convert the CHF commission value to GBP.            
                commissionGBP = baseCcyCommission / MarketData.GetSymbol("GBPCHF").Bid;
            }
            else if (baseCcy == "AUD")
            {
                // Use GBPAUD to convert the AUD commission value to GBP.            
                commissionGBP = baseCcyCommission / MarketData.GetSymbol("GBPAUD").Bid;
            }
            else if (baseCcy == "CAD")
            {
                // Use GBPCAD to convert the CAD commission value to GBP.            
                commissionGBP = baseCcyCommission / MarketData.GetSymbol("GBPCAD").Bid;
            }
            else if (baseCcy == "NZD")
            {
                // Use GBPNZD to convert the NZD commission value to GBP.            
                commissionGBP = baseCcyCommission / MarketData.GetSymbol("GBPNZD").Bid;
            }

            return commissionGBP;
        }

Any idea how to run this in backtest? Is there a neater way to calculate commission?

Thanks,
Marc