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Typically for a comparison to be useful, you need to compare like to like. In this case, you've got a bit of an "apples vs. oranges" situation -- you have the IRR for one item, and the opportunity cost for the other item. You really can't say unless you know the same metric for both things that you're comparing.
There are many techniques used in project selection. one of which is the opportunity cost "This is the cost that we are giving up by choosing some other project". if we have multiple projects to choose from, we will choose the one with lesser opportunity cost.
It is worth mentioning that opportunity cost involves many factors other than just the projects price.
This interview with Simona Fallavollita (LinkedIn Profile) was recorded at the magnificient Project Management Institute (PMI)® Global Conference 2017 in Chicago, Illinois. We discuss the how, what, why and when of the changes that are coming to the PMP exam.