Calculation of Trading Costs

Does anyone know how trading costs are calculated? Last week I placed about about 15 trades in a new system I’ve just introduced. All stocks wee highly liquid. On that day my adjusted and unadjusted performance graphs diverged by about 0.5% (around $100 on a portfolio of $20,000). I’ve seen somewhere that it InteractiveBrokers fees are used, in which case direct transaction fees should not be more than $1. I bought small quantities of highly liquid stocks so I can’t imagine it is because of an estimation of market impact since this would negligible, it also shouldn’t be my own fees since I don’t currently charge any for the system.[LINKSYSTEM_81894127]