Much debate here. Nice to see everyone so passionate, all with an eye toward causing everyone to do a little critical thinking. At the end of the day, you have to do what works for you. If it keeps on working for you, you won't change it. If it doesn't, you will.
First, the discussion of "what is equity." Seems to be that some people choose to use cost because they say all the talk of buying "below market value" is nonsense. They say, if it's what a buyer will pay, that's the market. Period.
Although I think there's some truth to that, the fact is that -- at least I hope it's a fact -- some of us are buying for properties at a cost that really is below their market value. Or, at the very least, we are buying properties where we can create value by making repairs and getting more than $1 in for every $1 out.
Case in point:
I bought a duplex for $115K. I put $12K into it. My basis is $127K. It rents for $700 a side. Let's assume the 50% rule holds, so it cash flows $700 a month.
If I use my basis to calculate equity (let's assume I paid cash), then my return on equity is $8,400/$127K, or 6.6%.
However, let's say I know (or think) that the property is worth $175K. That is, I feel that I have created $48K in equity for myself because (a) I bought the property right to begin with and (b) I've just got mad skills at renovating places cheaply. If I really think the place is worth $175K, and assuming that I'd pay about 9% in transaction costs (commission, closing, etc.) to sell the place, then my "real" equity in the property is:
$175K - 9% = $159,250.
So therefore, if calculating an ROE on my investment, I come up with $8,400/$159,250, or 5.3%.
I think it's completely correct to look at it this way, IF my "market value" is accurate, because you need to know what your investment options are and if they are better than what you have now.
It's like if someone gave me a $100K house for free, and it rented for $1 a year. Sure, my return on equity could be considered infinite since my basis is zero. Why sell? I'm making an INFINITE return on my investment, right?
Well, of course you'd sell that place in a New York minute, because you could take the $90K net from the sale of that place and make more than $1 a year from it.
Another comment on the notion that what you pay IS the market, if that's the case then I guess wholesalers should just give up the game, shouldn't they? I mean, their whole business model is predicated on buying (or tying up) a property for below what someone else (hopefully a rational person) will pay for it.
So, for me...
Return on investment = the net income before debt service divided by the entire cost of the asset
Return on equity = the net income after interest expense (but not principal) divided by your equity (using whatever number works for you)
Cash on cash = the net income after debt service (including principal payments) divided by how much actual cash you have in the deal
There are many ways to tweak these numbers using accruals, etc., and that's where you have to think for yourself.
Oh, as for GAAP, as a "big-bank" trained credit analyst, I know that GAAP, in the literal sense, is far from perfect. It allows earnings and numbers to be massaged to meet a company's needs. In summation, we went to great extent to deconstruct and reconstruct financials to get to the soft, chewy center of what is really going on.