Using bank funds to increase your ROE.

Even taking your CF number that returns ~4% Return on Equity (ROE) for the $200k you have in there.

Return in Asset % (ROA)/(1-LTV) = Return in Equity (ROE).   

In summary, only walk away from a clear winner if it's absolutely necessary to get you to an exponentially better (and very low risk) winner...but, that's just my own opinion and your mileage may vary...To your question about whether there's a particular formula to help you decide whether to keep a cashflowing property vs. sell it, I'm not aware of a specific formula for that (but, I'm also not an accountant or a finance wiz)....however, when I hear investors discuss this issue, it seems like they often reference return on equity, and mention how RoE is one metric that's important in their decision of whether to keep a cashflowing property vs sell...

The metric here is return on equity or ROE. 

I'd calculate your ROE and see if you can get something better in the same area.

It will cut into their ROI and ROE numbers but they won’t be losing money.

I've seen people focus on cash-on-cash return, ROE, ROI, etc. , but I was curious to see what metric you guys typically pay attention to when looking at a deal. 

I think instead of mortgage payoff, you may calculate your return on equity (ROE). 

Our ROE is far higher than we would ever achieve as passive investors, and although it requires perpetual, active participation, it's been highly rewarding in comparison to our individual and collective experiences as real estate investors.