I realize this isn't quite the same, but it relates. If you are trying to value OF vs P, look at the FA contracts signed by players at those positions the past 5-10 years. Without actually digging in and looking, my guess is many more pitchers have signed $50M contracts than OF. Doesn't that indicate that teams value SP over OF? That is why we can't get Shields for Span alone. Having Bourn and Upton on the FA market doesn't help Span's value, although he should be a cheaper option.
He does rake on 2-0 and 3-1 in terms of batting avg but I am convinced that after you account for the probabilities of strike and ball sequences, that Revere is actually having a slight negative effect on his OBP. And then you can further account for the hits he is not sacrificing due to him being still a very effective hitter in 2-1 and 3-2 (though less in 2-2). Again, I will go through the math which factors in his plate discipline and the percentage of non-swinging strikes he is actually seeing, and how these iterations would play out, and probably make another blog post to clarify this hypothesis.
OK, so I may have been wrong. A quick look (I may have missed a couple) on Cot's shows 11 OF and 9 SP signing $50M+ free agent contracts. There were however more SP signed to $50M extensions (8-4) - meaning they signed with current team before reaching FA. There just seems to be more team looking for SP than OF.
I agree with someone earlier that said that if we trade Span we are giving away an above average OF, so we should get an above average pitcher in return. I hope the Twins stand firm with that. That is probably why we haven't seen a trade, yet. People are probably trying to get him for a 4th or 5th starter. I think it would be a shame if we don't at least get a 3rd or better. ML avg. ERA for starting pitchers was around 4 last year, so I would like to see a lower than 4 FIP for whomever we get.