As of now, unless something changed in the last 24 hours, Dallas has to trade for him (folks seem to be forgetting this). On top of that, his salary is bananas, and that's at a time that the Cowboys don't have a ton of room to work with unless they let some other starters go. Then there's the Dez contract situation. Finally, there's his age, and the mileage on the guy. I'm sure his deal would be restructured, but you're also talking about attaching the team to a guy for something like 5 or 6 years, knowing that the back end has some serious dead money.
If Dallas was an elite RB away from winning it all this year, I might get it, but that wasn't the case. If you go with AP, you ask Dallas to get weaker elsewhere, and make no mistake, getting rid of Free and Carr right now does not make them better no matter how much some of us dislike those guys. Also, forget about the modest contract for nice to have players. With AP and Dez on the team, that dream is gone in 2015.
So again, it's not realistic to me once all factors are considered. Impossible? Not at all. Smart business? Not even close.