Power and Cooling Cost compared with Server/Hardware Cost

1. You need to convert BTU to watts.
2. Convert Watts to kilowatts
3. Then multiply the kilowatts by whatever you currently pay the electric company per kWh.
4. $$$ Profit $$$

You should have two sets of numbers: the electricity used by the systems (880W in your case) and the electricity used to cool them (convert Tons to BTU to Watts to kWh), then add them up.

You not only have to account for the electricity you use to power the systems, but also the electricity used to cool them.

You computed the cooling you need not necessarily the cooling you provided. These can be two separate numbers. If your ambient temperature was around 70F these numbers are close enough.

You're on the right track in terms of ventilating the hot air to the outside. You need to cycle fresh cool air in to the room\cabinet and exhaust the hot air out of the room\cabinet. You do not want to recycle the same air around and around. The key is to maintain airflow through the room: bring fresh, cool air in and send hot air out. My opinion would be to use the exhaust duct in the attic to send the hot air to the outside rather then sending it into the attic. If you have adequate airflow you may not need a portable air conditioner. Try it for a few days\weeks with just exhausting the hot air out of the room and see how that works. If it's still too hot for your comfort level (from the server's perspective) then go ahead and invest in a portable air conditioner. Be wary if the air conditioner has a water collection tank that you'll have to manage the emptying of it unless you can hook up a drain hose to it and sent it down the storm drain\sewer.