Taken me a while to "get it" but agree with @Brian Burke, cap rates are relatively meaningless when you are underwriting the PP of a property because of all of the variables.
We will underwrite the building using solid/conservative information on gross income (what are market rates, can we raise rents over 2-3 years, what reno needed to get to market, how do are units stack up compared to others, what is market vacancy, etc.), what we can achieve not what the current owner has done. Sometimes there is upside, sometimes there is not.
On the expense side, same thing. Conservatively underwrite based on a combination of actuals and market knowledge. Can we lower utility costs with energy efficient fixes? Are taxes going to go up in a couple of years after purchasing the building? What is the per unit expense cost? Is the current owner just bad at running a building?
Overall, the idea is that you might be buying a building at a 6% cap rate (or less) but when you underwrite conservatively using market data that you know and are comfortable with, you can run the building better than the current owner, make the building cash flow and effectively making it maybe a 9 or 10 cap building.
A good example. One of the buildings we're buying is a 29 unit on the south side of Chicago. The OM was something like a 7 cap on actuals and a 8.5 cap on proforma. All of this is BS information. In reality, the current owner is losing a lot of money every month. He's in a death spiral because he doesn't want to put more $ in to the building so his occupancy is dropping (around 65% right now) and his tenant quality is dropping. His current strategy is not working and will never work. Working with a PM that specializes in this area, we have a strategy that involves complete reno of units, combo of section 8/market tenants, getting rents to market and offering a quality living space that attracts better tenants.
Long story short, we're technically buying at a -7% cap rate but feel comfortable being able to stabilize this asset within 3 years to have it running at around a 9% cap rate (based on our PP) with upside. Current cap rates on stabilized buildings in this area (Jackson Park) are around 7%, just to give perspective.