That's a question worthy of further exploration, no?
Since 2006 (per BB Ref):
Dunn: 787 G, 3266 PA
Fielder: 677 G, 3386 PA
Dunn: 99 Runs Above Replacement (RAR); 20 RAR/YR; 10.2 WAR Total
Fielder: 152 Runs Above Replacement (RAR); 30 RAR/YR; 15.6 WAR Total
oWAR (Wins above replacement, without concern for fielding. Offensive measure)
Dunn: 17.0 Total; 3.5 AVG
Fielder: 19.4 Total; 3.9 AVG
REGULAR STATS:
Overall
Dunn: .252/.378/.524/.902, 1407 TB, 132 OPS+
(Per-162): 41 HR/105 RBI/ 292 TB
Fielder: .279/.386/.536/.922, 1555 TB, 140 OPS+
(Per-162): 38 HR/107 RBI/ 316 TB
I don't think those would justify $100m more, but Fielder being young and very consistent in his own right will probably warrant a bigger payday.
Dunn is really bad in the field, and that's where his reduced value comes in. If either he or Fielder would be the DH, their offensive differences won't be worth a huge difference in pay. If they are hiring a DH, Fielder is who I would prefer if $$ is not an issue.
Adrian Gonzalez is my preference above both, obviously