The posting fee doesn't really affect the contract signed. You can say, obviously, that Darvish was worth 113 million to the Rangers over 6 years, but you could also say he was only able to leverage a 10 million per year contract for himself.
Now, a team like the Yankees (who will almost assuredly get Tanaka now) may be willing to offer a bigger contract, but with the lower posting fee it will still come out better. If they signed Tanaka for 6 years, 15 million per, and 20 million posting fee, he'd still be a few million cheaper than Darvish. Teams with enough cash are bound to do something stupid, but is any team really going to shell out top line starter money for someone who has never pitched in the majors?
Part of the reason posting fees were getting so high is because that money doesn't count against payroll. Deep pocketed teams could throw as much as they wanted without getting into luxury tax "trouble". I don't think the new cap is going to limit the any competitive edge for large market teams, but the system needed to be changed for everyone's sake anyway.