Overfitting: Quantifying the Cost

I've been struggling with overfitting for a while now. I've tried increasing the profit-risk ratio and using walk-forward optimization, but I still feel like I'm losing money to the beast. Has anyone found a way to quantify the cost of overfitting in terms of expected returns? I'd love to see some backtesting or statistical analysis on this. I've been trying to use the profit-risk ratio as a proxy, but I'm not convinced it's enough. Any insights would be greatly appreciated.