I agree that it's not enough, but in this case Cook is not being traded for a pick. He's being traded along with a first to sweeten an offer to move up in the draft. The Bills would be trading Cook AND 30 for 12. But I agree that won't get you 12. It could get a pick in the 20's, but teams that need a RB would have to weigh paying the proven Cook over drafting the cost-controlled Hampton. I think Denver at 20 would be a good spot to look. That places Cook's value at a high 3rd, according to the draft value chart. But Dallas might also take 30, 56, and Cook for 12.
https://www.drafttek.com/NFL-Trade-Value-Chart.asp