You'd have to pay a percentage of proceeds to the authorities in order to monetize staying an authority over selling your reputation for a guaranteed win in bitcoins.

Depends on the authority type. The pre-agreed format type of authority shouldn't need paying; nor one that relies upon mass voting to make decisions. Because in neither case is there a small group of controllers with the power to 'fix' the result.

One that uses a panel of experts will need some way to prevent a cabal of experts colluding together, and some form of reward for their time. The reward might be the reputation they gain from being seen as an expert (like with WikiPedia). The anti-collusion measure might be by having quite a large panel and selecting 7 members from it at random for each decision (preferably, selected only after the bets have been laid). However, yes, the system ought to be set up in a sufficiently flexible manner to allow each authority to be able to set in advance a rate of 'cut' for bets using them as the decider (with competition between authorities keeping such rates low).