Building a rig with 960 4gb x4

Hi guys,
Sorry if im asking a question that might have a few 100 similar topics, just could not find what I was looking for.

First of, my goal with mining is not to become a millionare in a year. Dont have the cash for that. However i am interessted in adding a few extra dollars to the beer account every month.

Thats the reason. Here is the question.

Building a rig with 4pc 960 4GB.
And use nicehash mining site. Based on their site,
A 960 brings in USD 50-55/month, based on avarge last 30 days and if the power cost is USD 0.2/kWh.

With this in mind. A rig with 4x 960 would bring in about USD 200/month.

With that calculation I will pay off my rig in 4-5 month and after that provide so I can update with new vga cards with in 12 months.

Im I thinking correctly? Or have I missed somthing?

Thanks alot for your help team!