Turnover and transaction costs in portfolio selection

Good morning

I am doing some performance analysis work between the markowitz model and a model with limited number of assets, and to make the experiment more real I am trying to include the transaction costs associated with rebalancing my portfolios. I have all the turnovers calculated in consecutive periods, daily out-of-sample returns and cumulative wealth available. For the in-sample analysis I have considered a time window of one year and an out-of-sample window of 20 days, using the rolling time window technique. How can I estimate a reasonable but approximate transaction cost?