Bourn is starting to get paid, now in his second year of arbitration. This means he has one more season of arbitration and then can become a free agent.
He's not expensive yet. Due only $4.4 million in 2011 after a couple 4+ WAR seasons, a player with his value would cost far more on the open market.
He's entering his age 28 season, which means he'll be 30 when hitting the free agent market. While players with his skill set tend to age well, the risk after 30 always goes up. It's quite possible he's already in the prime of his career and could start to decline in his early 30's.
To make things worse, he just added Scott Boras as his agent. Scott Boras clients don't sign contract extensions.
There are two things the Astros, a team in desperate need of prospects, could do. They could hang onto Bourne and hope he improves on his 2010 numbers or try to maximize their return now.
I personally think Bourne stands for a small amount of regression. This doesn't kill his value, but it doesn't improve it either. Teams prefer as many seasons of cheap club control as possible, so obviously two is better than one.
Adding Boras as his agent could get the Astros thinking about seeking a big return now while Bourne is young, cheap and under club control.
Then again, the Astros are as bad as they are because of the thinking of those in charge.