It should be obvious, but aparently it's not. Most new investors are so hung up on getting a "good deal" and are willing to considered a wired or undeireable property, just to get a good deal. And even if the "good deal" comes at the cost of beeing not a good property. 

I was no exception in my early years. The fallacy is even more obvious with residential buyers: they feel good about a deal as long as they can negotiate down from list price. Even if they are still over paying. On the flip side they feel bad about offering more, even if that is still withing fair market value.

The longer I do this the more I think of my real estate as a collection. And today, I evaluate properties more on how desireable they are (or can be made) than how good of a deal I get looking at present value. Once you can look back on 10 or 15 years of investing you realize a few things: 

1.) you always feel you are paying too much. 
2.) 10 years later you don't even recall how much you paid
3.) the year later you wish you had bought more for that price
4.) what really matters is that you have bought GOOD properties and locations

I believe it is a mistake to buy based on cash-flow only. "Run your numbers until you find a good deal" is horrible advice when considered in isolation - noobies will buy a bad property, just because the cashflow is $200 better than a desireable property that would benefit them so much more in the long run. And sorry to say this, if you really "need" $200 every month, you should not be investing in real estate in the first place!