He's a great back, IMO best in the NFL right now. Vision, power, speed, agility ... the total package.
Otoh I cant really say the cowboys offer is that far off. Its a RB by committe league now, and given the right oline many backs will be successful running behind. Plus factor in RBs low age ceiling and injury risk, and it just doesn't make sense to risk paying too much ... I'd say best case he gets $24mm, with $17.5mm guarantee.