I think this is the general sentiment of Cowboys fans. The Cowboys are going to need the cap space for Dez and several other players (I am thinking Rolando McClain, Bruce Carter, Doug Free and Ron Leary), so I doubt they will be able to sign Murray at his market value.
Keeping Murray fresh for December and a possible playoff run is probably the biggest concern. Of course they might not make the playoffs if they don't use Murray and running game, but I think they have capable backs that could spell Murray for a couple of series, which would then lower his total carries to a little less than 400 for the year.