Dexter Fowler is the most valuable non-core asset the Rockies possess right now, and as such, there's been a lot of talk that he might be on the trading block. The Rockies have indicated that they want to get young, Major League ready pitching in return for the center fielder. Since Fowler hasn't gone anywhere yet, it stands to reason that the Front Office hasn't been offered a package they think is acceptable.
But what can we really expect for Dexter? He's under team control for two more years, at prices that are no longer particularly cheap ($7.35 Million in 2014, final year of arbitration in 2015). Dexter is a very skilled player, but it's hard to gauge how much other teams would give up for his services. There are, though, a few comparable players traded in recent memory that might shed light on the subject.
Span was traded from the Twins to the Washington Nationals before 2013, and he's probably the closest comp to Dexter. He's a speedy center fielder with above average walk rates and a reputation for good defense. He had three years left on his contract before the trade at slightly less money than Fowler. Dexter has more power and is two years younger than Span (so if the trade happened this winter, he'd be one year younger than Span was when he was traded).
Span averaged 5.5 WAR in his last two seasons with the Twins, while Dexter averaged 4.7 (defensive metrics loved Span while they've always dinged Fowler; I don't know if Front Offices would see it the same way). Overall, I'd say the two had pretty similar values at the respective times when they were on the trade block.
So what kind of return did Minnesota receive for Span? Well, it's hard to say yet: the player coming back was Alex Meyer, an A-ball starting pitching prospect. Meyer was ranked the 59th best prospect in baseball at the time, had good stuff, and became the Twins' #4 prospect (per Baseball America). Basically, the Twins received the Nationals' version of Chad Bettis.
Meyer is a very good prospect, but he hasn't cracked the Majors yet and probably won't in 2014 either. The Twins were probably on a different time frame than the Rockies (who want to win now); they could afford to take a chance on a guy still a few years away from the big leagues. The Rockies obviously want someone who can slot into the rotation immediately.
Rasmus isn't a great comp, since he was younger, pre-arbitration, and supposedly had personal issues with the Cardinals (ie, Tony La Russa). Additionally, the Cards were in the midst of a playoff run and needed rotation help immediately. Still, Rasmus was a young, skilled, successful center fielder, much like Dexter.
Jackson was a good pick up for the Cardinals, but he was a rental, and the relievers were nothing special. That wouldn't be the kind of package the Rockies are seeking for Fowler.
Neither of these are perfect comparisons, but they provide some context regarding trades of non-star center fielders. The returns have been more modest than Rockies fans would probably like (an A ball prospect or a rent-a-pitcher with relievers). While we want to receive immediate, substantial rotation help, history suggests that that might not be forthcoming.