Of course Fielder would take less money to play for the Rangers. I don't think there is any real mystery here. Top FAs go after two things - maximum guaranteed money, and the chance to win a ring. Any FA is going to prefer to play for a team that is ready to win it all. But they are going to be swayed by significant money. To be lured by a lesser team, they would need to get a big bump in overall dollars - usually in the form of extra years. That was Werth. The Nats got him because they went beyond the market by probably close to 20% in contract value. If the difference isn't huge, there is no reason to turn down the contender.
The Rangers have been mentioned as a natural home for Prince since the get go. But Darvish adds a wrinkle. If Darvish isn't a roadblock, they likely would have already signed him. If they don't sign Darvish, they sign Prince. If they do, they don't, and he looks to plan B. If the Nats have an offer equal to the Mariners and Orioles, the Nats have a better chance of getting him, because they could be seen as much closer to contending. If the other suitors offer a Werthesque differential in overall contract, he'll follow the money.