It's closer to 22.5% in total. [1] But it also depends on specifics like race, location, type of bet [2]. Looks like a conventional win, place, or show bet has the least taken out of it

[1] http://www.calfairs.net/files/publications/14.pdf -- check slide 5

[2] https://www.scribd.com/document/84887435/CA-Authority-of-Rac... -- An Example for Northern CA breeds on page 10