Teams are valuing draft picks more and more these days due to the salary cap, but the main problem with trades is the cap hits most teams take due to guaranteed contract money acceleration. Teams either have to have serious cap issues or are in rebuilding mode (Eagles for example) to want to absorb an abrupt cap hit by releasing or trading a high salary player.
The fact that the Cowboys's drafting has improved over the last few years is evident because of the focus on retaining their draft picks beyond their rookie contracts. A lot of fans focus on "free agent" signings as only the ones that come from other teams. However, the Cowboys have resigned would-be free agents Sean Lee, Tony Romo, Orlando Scandrick, Tyron Smith, etc. and also re-signed actual free-agent, Dez Bryant.
When you sign a free agent that played for another team last season, there are many variables that come into play. There's the Spagnola's mantra of "If they were so good, why did their team release them?" There are also concerns such as will they fit into the Cowboys systems and schemes? Will they continue to play at the same level or better than they did the previous season with their former team? Do they have a history of injuries and just had one good year (which is why a lot of teams didn't even talk about signing Murray last year)?
The best teams are focusing more and more on building their teams through the draft and even rookie free agents, and only filling a couple of holes using free agents.
The other issue is that teams don't know what they are going to get in the draft, so they are hesitant to sign free agents when they may end up filling that position with a capable drafted player.