For the three seasons 1998-2000, Major League right fielders held runners to taking an extra base about 48% of the time. In other words, when runners were on first or second and a single was hit or on first and a double was hit, 48% is about how often they took the extra base.
The best right fielders over that span were Raul Mondesi (36.3%), Bobby Abreu (36.8%), Manny Ramirez (41.4%), Jose Guillen (43.9%) and Vladimir Guerrero (44.0%). The worst was Matt Stairs (56.4%).
The right fielder with the highest number of opportunities to stop/allow extra bases in 2000 was Jeromy Burnitz, who faced 157 such opportunities. A few other right fielders faced over 100 such opportunities.
In any event, if you take the difference between the best (Mondesi) and worst (Stairs) on the list (56.4% - 36.3%) and multiply by 157 opportunities, you get a difference of about 32 extra bases. If you compare Stairs to the average (48% - 36.3%) and multiply by 157 opportunities, you get a difference of about 18 extra bases.
So that should give some idea of the order of magnitude, at least for that season. The league's worst-throwing right fielder allowed about 32 more extra bases than the league's best, and about 18 more extra bases than the league average, given an equally high number of opportunities.
I wouldn't be particularly pleased with Pence allowing 18 extra bases more than the league average, or 32 extra bases more than the league best, in 2008, but I'm not sure those figures are catastrophic, either.