I have not read this term used in discussion often on this board although opportunity cost has become trendy. I think the concepts work together as an opportunity cost is neither good nor bad unless compared with marginal gain from a prospective transaction. When adding player A, the marginal gain is the production the team receives from player A over the default option for the team had they not made the transaction. The opportunity cost is that separate action prevented or forgone by acquiring player A.
Let's look at the proposed Fuentes FA acquisition. The marginal gain is not how much better Fuentes makes our bullpen over last season. The marginal gain is how much better will Fuentes be than the inhouse options of Perez, Motte, Kinney. There is an argument that we are moving these three down a slot in the bullpen, thus, Fuentes is really displacing a guy like Brad Thompson. Reasonable minds can debate it but the marginal gain is the prospective bullpen with and without Fuentes. I'm not a fantasy stat guy so I'll not debate win share and other metrics.
The opportunity cost is the lost 1st round draft pick and the player that the Cards could have acquired with the $10 mil annual salary going to Fuentes. If I say this is Ben Sheets, you may retort that he will cost more than $10 million per season. True but that is not the test. If the Cards forgo signing Fuentes, it is possible they may find modest payroll savings at other positions to put them into a position to acquire Sheets. For instance trading Rick Ankiel ($3 million) and replacing him with Rasmus (league minimum). There is not a way for the Cards to wangle the cost savings to acquire both Fuentes and Sheets but I think they could find the money to land Sheets alone. That's an opportunity cost. Then calculate the marginal gain to the team of Sheets in your rotation versus the dumpster dive for a Dave Duncan reclamation project that will occur if we sign Fuentes. Of course, it's possible JMo could pull another rabbit out of the hat as he did last season with the Lohse deal or signing Fuentes frees up McClellan to go to the pen.
Reasonable minds can differ on the facts but I think this has to be the analysis. I'm sure most people on this board already make these assessments mentally when sizing up a deal ... but I wonder if guys like Strauss actually sit down and think about it.