He's regressed each season since his 2006 campaign when he went 40/40 (46/41) with the Nationals. At age 36, it's highly unlikely that he'll ever produce at that level again.
Since he is off to a slow start this season (.212/.236/.212, 0, 8), it may be best for Theo Epstein and Jed Hoyer to consider trading Soriano this year—rather than next year, or the year after that. His value will be highest in the next few months, as age will continue to catch up with him.
It will be difficult to move his albatross contract unless the Cubs pick up a large chunk of it, but some teams would definitely be interested.
Soriano still displays some power—he's hit at least 20 home runs every season since 2002—and he is capable of going on hot streaks that can help boost a team during the season.
Epstein and Hoyer are dead-set on rebuilding the club—as they should be—and capitalizing on trading Soriano can only help the process. The return wouldn't be great, but it could improve if the team is willing to give more money to get rid of him.
Soriano will make $18 million each of the next three seasons. If the Cubs pick up about $5-6 million per season of that, they could expect a mid-level prospect in return. That'd be the minimum, as no team is likely to take Soriano for less. Should the Cubs be willing to pay up to $9 million per season, they could receive two mid-level prospects or three lower-level prospects.
The Cubs need to rebuild, and if they have hopes of competing in 2015—or sooner—they'd be wise to unload the massive contract of Soriano.