I will just give you my opinion on the matter which is far less scientific, and in the end, probably worthless.
When I am looking at the value of a position, I start with snaps. Because the NFL is essentially 11 players trying to win 11 one on one battles, the amount of snaps played matters most to me. If the player/position I am looking to draft doesn't play a significant number of snaps, then I downgrade it. This is why I put such an emphasis on offensive linemen, and the importance of having great players on that unit. They effect the success of almost every offensive snap, so why wouldn't you value those positons most?
The 2nd item that I look at is, how does that player's positon effect the QB, either team or opposing? This is a reason I feel S's are underrated. By being the last line of defense and plaing in the middle of the field, they are generally the QBs first read, and can directly influence where the QB decides to go with the football. Also, by being in position to prevent big plays, they can lengthen the amount of plays the opposing offense will take to score. By rule, the more plays in a drive, the more opportunity for an offensive mistake. OGs directly effect both the running game and the passing game, which makes them very valuable as well.
The 3rd item is, how does that position effect 3rd downs? I beleive 3rd down is the most important down in football in today's NFL. It either extends or stops drives, and I believe a teams ability to be successful on 3rd down in relation to both sides of the ball will dictate their success as a team. So, if I'm drafting a player, I want the player to make a difference on 3rd down most of all. This is why I believe quantity can win over quality when it comes down to CBs and WRs. You can always double a team's best WR, but it's how your #3 or #4 CB match up against the opposing teams 3rd or 4th receiving option that can dictate the outcome of the play.
As far as how this all goes into player evaluation, which is why you hear teams reference to where players rank on their board. I could argue that the the 3rd OG on the Cardinals draft board should be Senio Kelemente who is tabbed as a 4th round or later player. This is how teams build solid units with later draft picks. They identify players who fit a "type" that fits their specific scheme and then draft that player a bit earlier than the league values that player. The Cardinals have struggled with this because they have had so much turnover in the organization it makes it hard to see what types of players fit best.
My final thought on the matter is if I have a definite need to improve my offensive line, and I feel that Devid DeCastro is the best player to do this, I draft him at #13. However, if I feel that Brandon Brooks is almost as good of a player and I can draft him in the 3rd round, I draft both players and am set at both OG spots for the forseable future.