Couple of comments.
First, great to post on this topic, which is so important. The key meaning of BV is knowing that you gave the customers something they really wanted.
I agree with your notion of doing a cost-benefit analysis, where the SPs or the complexity for something is a proxy for the cost. I would use Story Points as that proxy (since we already have ‘em).
I also favor using “Priority Poker” to make visible the relative BV of stories. Takes into account all definitions of BV. And works just like Planning Poker, except about BV. Mike Cohn favors this term (“Priority Poker”).

Having both, one can do a BV/SP factor on each story. The “highest” ones probably go to the top of the PB (some human judgment to assure the stats did not lie to us).

Your last statement is the most important: Having a clearer and clearer idea (today) where the BV is hidden (today) is ever more critical.

Thanks!
Joe