He'd have to take a big pay cut to make it worth signing him over DM. That's IF he is no longer a member of the Vikings, who still have him under contract.
I think this really is a smart move by the Cowboys. Sure, Murray had a great year, the first year of his career that he was healthy the whole time. Huge amount of carries, and historically, guys not named Eric Dickerson suffer a huge drop off in production following those types of years, when they have over 400 touches. Signing him to a huge contract would be silly, IMO. Just ask the Titans and the Chiefs after Chris Johnson and Larry Johnson had their historic years.
This year's draft is very deep for running backs, and the Cowboys may even have their pick of Gurley or Gordon at 27, as rb has been a very devalued position in the past few years. If not, they could still get a top 10 back in the 3rd or 4th round. Plus, there is a pretty good crop of FA running backs available before the draft, who will be much cheaper than Murray.
That said, I can see Murray coming back. He deserves to get what he can, but with the above mentioned circumstances, his market value may be down compared to what he could have historically gotten. He wants to be a Cowboy, and if the Cowboys are close to what he could get elsewhere, I think he would sign with them. Both sides want to get a deal done, and both sides know that pro football is business.