Benefit = the total amount of savings (proprietary price - open source price)
ROI = benefit / cost
To calculate the ROI of a particular migration, you need a few cost numbers. Whenever possible, use historical budgetary data for the best accuracy. Not everyone pays the same for software, so if you use the actual value your organization pays, your calculation will be more credible.
You need to find out two figures: the cost of the status quo replacement system and the cost of deployment of your open source alternative. Calculate the cost of the status quo system as everything your organization will pay if you don't go with open source. The cost of the open source deployment should not be $0. It's neither believable nor true. Be sure to include support contracts from a third party and outsourcing costs for deployment or staff training, even if you pay nothing for the actual software.
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