Go through the NFL and look at the RBs. How many of the top rushers were first rounders. You love doing this with QBs, so why not RBs.
Murray - 3rd Round
Le'Veon Bell - 2nd Round
McCoy - 2nd Round
Lynch - 1st Round
Forsett - 7th Round
Foster - Undrafted
Lacy - 2nd Round
Hill - 2nd Round
Gore - 3rd Round
Lamar Miller - 4th Round
Alfred Morris -6th Round
Matt Forte - 2nd Round
Jamaal Charles - 3rd Round
So basically only 1 1000 yard rusher last year is a former first round pick and that was almost a decade ago. So why in the hell would anyone be spending first round picks on RBs? Hell a lot of the guys weren't even second rounders.
First of all since the NFL changed after 2005 or so there remain only a few contending teams built around running the ball anymore-- Dallas along with the Eagles, Ravens, Steelers, 49ers, and Seahawks. Maybe the Saints and Panthers, and now the Bills too. This NFL-wide de-emphasis artificially pushes talented RB's into lower draft slots alot like the way tweener OLBs were before 3-4 defenses became the rage.
The 2008 draft was the deepest I can remember at the position, and guys lasting til the late 2nd and 3rd rounds would have gone much earlier in other drafts, and that also pollutes these numbers a bit.
The Cowboys had McCoy rated as a 1st round talent, and "The Deemphasis" slid him the 2nd. Gore was one of the top prospects of all time until his college injuries which dropped his draft status-- and he was a steal in the 3rd round. Bell, Rice, and McCoy were 2nd rounders, but they were not late second rounders which is what Dallas will have to use. Charles is a fine player, though I feel not powerful enough to be a sole bell cow type RB to last five years. It's outrageous that Eddie Lacy fell into the 2nd, but that's the state of the NFL.
The Forsetts and Millers of the world can pinch hit in a given season and the Morris's can run well enough from a pistol (though maybe not pass block or catch the ball), but would you have banked on it? Do you think the Ravens will bank on Forsett long term? Or that Washington views Morris as a true franchise player independent of a gimmicky offense?
Dallas tried to pay a college part-timer taken in the 4th round like he was a full time stud, and they got burned for it. They bought into the hype that another part-timer was a 1st round talent, and they got burned for it. They need a sure thing, and likely a sure thing in the NFL was probably a sure thing at the college level too and not some part-timer like Barber or Felix.
The Cowboys' 2nd rounder is all the way at the bottom. Sure Dallas can get a fine back there, but can they get a lynchpin franchise back to ride into the playoffs year after year? A guy who can block, catch, and has the toughness to play 3 downs? Maybe, though I'd say their chances were far improved if they use their bottom 1st round pick.
Last thing, I think the trend is going to reverse itself and more teams will return to leaning on RBs to help their QB's out. Anyone who watched the effect on Romo can't ignore the benefits. That's going to drive the cost back up.