My understanding is that there are no financial rewards from the FIA, for winning the WDC. However, the Kudos of winning the title has major financial benefits.
When it comes to renegotiating sponsorship contracts, RBR can ask for more money. They can ask their sponsors to pay more OR there are plenty of other sponsors waiting in the wings ready to pay up, wanting to be associated with a winning brand.
The more championships/prizes/medals/events that a team/sportsman wins, the easier it is to market that team/sportsman and the easier it is to get sponsors to cough up more money.
If you look at somebody like Tiger Woods or Roger Federer...sponsors what these guys to endorse their products because they are winners. Plain and simple. Everybody (except for the Brits, who seem to adorn losers and underdogs), likes to be associated with success and winning. If RBR win the WDC, it will improve their standing from many different angles.
One argument against having Webber winning the title though, is that if their golden boy - Vettel - gets beaten by an average driver (which Webber is), it won't do well for Vettel's standing. Imagine the following conversation between a prospective sponsor and an RBR representative:
RBR: Yes, we have a fabulous new driver line up for 2012, with our man of the moment, Vettel, leading the team. We also have a new driver who we think will do great for us, who has replaced Webber. We believe we have one of the best cars for 2012 and as such believe we will be winning everything. On this basis our sponsor ship prices are a little high, but as we will be winning so much and sponsors will be getting a lot of TV time, we think it'll be more than worth it.
Sponsor: Oh. Vettel. Wasn't he the guy who got beat in 2010?
RBR: Erm Yes.
Sponsor: ok, but he got beat by Webber who isn't really regarded as that great.
RBR: Erm Well.
Sponsor: So, if he got beat by Webber, how is he going to beat Alonso's Ferrari or Hamilton's McLaren? I mean, why shouldnt we offer Ferrari or McLaren, the same money you are asking, so that we can get our name printed on their cars? Surely they stand a better chance of winning than you...right?
RBR: Erm...Vettel has improved.
Sponsor: But, why should we invest in your team, as opposed to Mclaren or Ferrari?
For RBR, it makes more sense to ensure that Vettel finishes ahead of Webber. He is their chosen one and Horner made a right balls-up during the middle of the season which has resulted in even employees from posting on message boards (CSI NUTS). Crazy.
Ferrari have strong management, who stand by their decisions. If Webber was at Ferrari, he would be told in no uncertain terms that Alonso is your team leader. If he tells you to move over. You move over. No arguments.