I don’t see how position enters into it. That is to say, positional scarcity and the ability to find an adequate alternative – either later in the draft or via free agency – are already factored in to the price that people are willing to pay to get a player. That’s why RBs are the most prized possessions, even though they score about the same as WRs and significantly less than QBs. So, if the market for fantasy football players was totally rational, then a $40 RB and a $40 WR should be expected to provide the same amount of marginal value to their teams.
Of course in one draft there are going to be idiosyncrasies that ought not to be generalized from, but in a large collection of drafts, taken together and averaged, those idiosyncrasies will *largely *be smoothed out – so, that on average Ray Rice goes for $52 while Steven Jackson goes for $39 is a pretty good indicator that one is expected to be of more value than the other.
I suspect that those who think this trade is really unbalanced have the Average Draft Position from standard drafts in the front of their minds: Andre Johnson is a mid-late 1st Rounder, while Mendenhall is an early 2nd Rounder, so swapping them seems really unbalanced. But why is ADP more indicative of actual or expected marginal, fantasy football utility than average auction price? Both types of drafts have features that could potentially warp the “market” and make their results inaccurate as a list of most-to-least useful. (What’s really weird is that I’m apparently like most other people: I would draft Andre Johnson ahead of Mendenhall in a regular draft, but going into the auction the other night I was definitely willing to pay more for Mendenhall; there’s a disconnect somewhere.)
It’s actually a really interesting subject. Why are some players worth more in an auction draft than a standard draft? Which (aggregate) result tends to be more indicative of a player’s usefulness? I have some unformed, totally untested ideas, but basically I have no clue.