That wouldn't make sense. The SP branches are meant to be self-reinforcing benefits within a general theme.
Opportunity cost is the amount of X you give up for Y. There is opportunity cost to going into Piety instead of Rationalism because you lose all the science buffs , some gold plus 2 free techs.
But some of that is also offset by the gold earned in Piety. The opportunity cost of Piety over rationalism is therefore W culture + X science + Yhapppiness + Z Gold (note the values could be negative). Opportunity costs must also be controlled for things you would have built anyways. For example, in Piety, Temples and Monuments give extra happiness, however, you are likely to build both because they also give culture which is critical to social policies. What is harder to measure is the relatively likelihood the AI buildinhg something else in your puppet cities because monuments and temples are relatively less attractive buildings to them without the +1

And the cumulative turns it took for the AI to eventually getting to building them.
Within each SP branch, there shouldn't be any trade offs as the idea is to complete the whole branch. Now, there might be reasons why you don't do this and start a new tree, but that usually has to do with non cultural VC approaches and the immediate benefit of opening a new tree ... saw the happiness from the order opener, outweighing some marginal benefit to unlocking a SP within a branch.
If by opportunity cost you mean 'negative' effects in Social policies that's not an opportunity cost. That said, nothing sotps them from making it such that unlocking Piety gives you a 50% RA science penalty instead of gaming 50% RA bonus from unlocking Rationalism, but negative effects like that have limitations. As Civ3,4 has generally borne out, penalties and suboptimal settings tend to drive compulsive behavior and bean counting, as well as a negative feedback loop. It's also much harder to balance all the trees with so many moving parts.
It is however easier to review the relative benefit of adopting one SP over another SP if you're talking about trading off science benefits from culture benefits and each choice add something different to your empire.