This represents quite a shift for a franchise that wouldn't match a four-year, $56 million contract for Chicago-bound Adam Dunn. The titanic nature of this contract becomes apparent when you compare Werth's performance in recent years to Dunn.
Let's try to be as generous as possible to Werth and look at his last three seasons; those are the only three times in his career that he's received more than 400 plate appearances. From 2008-2010, Werth hit .279/.376/.513 with 87 HR, 251 RBI, and an OPS+ of .132. Over the same period of time, Dunn hit .255/.380/.526, with 116 HR, 308 RBI, and OPS+ of .138.
So what are the Nationals getting in return for the extra three years and $70 million that they're not getting for Dunn?