In horse racing where the bets are done via parimutuel[1], the challenge is that there's an upper bound on how much you can "invest" in a given wager without changing the odds. This makes it less likely that an expert will seek outside investment unless their model has them betting "wide" (lots of small bets vs. a few large ones).

[1] https://en.wikipedia.org/wiki/Parimutuel_betting