Baseball Info Solutions tracks the number of bad throws a first baseman saves, as well as the number of good throws he misses. According to John Dewan’s latest Stat of the Week, the difference between the best first baseman (Casey Kotchman, nine scoops above average) and the worst (Joey Votto, six below average). If you spread these out over a full season, you might say that the difference between the best scooper and the worst is about 20 saved throws.
The difference between a single and an out is roughly .6 runs. 20 times .6 is twelve runs. So twelve runs—a little more than one win—is a rough estimate of the difference between the best and worst first basemen in this one particular aspect of their fielding skill.
This is all extremely rough, of course, but it’s probably fair to say that the impact a first baseman’s ability to save (or miss) errant throws is generally worth a little less than the impact of an outfielder’s arm.