http://en.wikipedia.org/wiki/Internal_rate_of_return
IRR used to compare different income streams with NPV=0
I'm not sure what you are trying to do. You have a return on investment based on what you have spent on the apt bldg and what you sell it for +/- any income/expenses. Objectively analyzing whether to spend 750/mo to get 10,000 equity each month then ROI=1330% makes a no brainer helluva deal. Based on this, you should hold on until retirement and then sell for several $billions.
Are you trying to compare this investment to something else? The return is not the issue, its the "what if's".