I don't think the back-up distinction is all that appropriate, though. You can very easily get two players 200 carries a piece with average overall rushing attempts, which is enough to justify significant investment. Especially considering you are starting off with a negligible overall salary cap investment in that area. I don't know that the competent is likely to be particularly fierce for Bradshaw, either. Teams are going to be looking at him in the same role the Dolphins likely are, as it's what he has done and his health concerns are going to probably eliminate the idea a team is going to give him 300+ carries.
Stay clear. Let Lamar Miller carry the load and bring in someone to compete with Daniel Thomas. The 2013 draft is loaded with guys like Jonathan Franklin-UCLA, Ray Graham-Pittsburgh, Spencer Ware-LSU, and Mike James-Miami being nice fits
I didn't realize Goodson got ejected in Week 17 this year. But then, Cary Williams damn well should have been ejected from the Super Bowl and I don't think anyone is suggesting teams should stay away from him because of that.
Goodson is the guy I see going somewhere with a chance to be the feature back. I think Bradshaw is smart enough to know he'll be better in a competitive time-share than in a situation where he'd be the guy.