internal void CalcMarketPrice()
        {
            var isOnsale = IsOnSale();

            if (isOnsale || parent.IsClosed)
            {
                // reduce price
                marketPriceModifier.subtract(0.001f, false);
                if (marketPriceModifier.isZero())
                {
                    marketPriceModifier.set(0.001f);
                }
            }
            if (!isOnsale && parent.IsOpen) //rise price
            {
                marketPriceModifier.add(0.01f);
            }
        }