Well, here's how I've been doing it (still learning):
cap rate = NOI / purchase price
where NOI = (gross revenue) - (all expenses excluding financing, e.g taxes, insurance, hydro)
NOI = Net Operating Income (e.g. what's left from revenue after expenses)
Because everyone has different financing costs, but everyone would have the same revenue and expenses... until they change something e.g. renovations, etc. but that's a different analysis.
So cap rate is a rough estimate at how much you'd earn with no financing costs and no appreciation, compared to taking the same purchase price and investing it for interest. E.g. a cap rate of 5% would yield 5% of the purchase price after expenses. If you bought a $500k property with a cap rate of 5% you'd get an NOI $25k/year, equivalent to getting interest of 5% on a $500k investment.
Then you subtract your financing costs from the NOI, and that would be your cash flow. E.g. if I had a $400k mortgage on that $500k property at 3%/30 years, the P+I would be just over $20k, so $25k NOI - $20k P+I = $5k left over.
I don't mean to talk down to anyone, I just wanted to make sure I understand this myself. If I make a mistake I'd prefer to be corrected than to lose money, eh?
So, I'm still a little uncertain what expense fall within the NOI and which don't. Clearly property tax does, and financing doesn't. I don't think property management would, and I'm not sure about insurance - these are things that would vary between owners, and I think the point of the NOI and cap rate is to compare properties without depending upon who owns it.
Since I'm looking to buy-and-hold a rental property that can carry its' own cash flow, I'm concerned with the cap rate. If that's possible.