With interest rates as low as they currently are, ROI is pretty hard to nail down.  There is a huge difference in interest cost between different situations, and with interest as low as it is, that makes a huge difference in what one can pay and still have a profitable investment. 

I have historically made quick and dirty calculations using 10% interest.  That is not reasonable.  Right now, 5% may not even be reasonable.  You can pay twice as much at 5% as you can at 10%, and if you go to 1% it is ten times as much.  

So much depends on your outlook.   And your financial position.