How much do monorails cost at Disney World?

Disney does not charge for rides on their monorail. They are included in the ticket price.

The monorails are complimentary; the monorail stations for the parks at Disney World are outside the theme park gates, so if you just want to hop on a monorail and ride around you don't even need a theme park ticket (though you will likely have to be a hotel guest, or at least pay for parking, to get to a monorail station in the first place) ... still, on your first (or last) day at the resort, if you don't want to use up one of the days on your pass for a partial day at one of the parks, you can still ride the monorails.

At Disneyland Resort in California, the monorail is also "free" (there's not an explicit separate charge for it), but:
1. There are only two stations.
2. One of the stations is in Disneyland Park itself.
3. Everyone must exit at the Disneyland Park stop; you can't start at the Downtown Disney station and make a round trip without getting off in Tomorrowland and reboarding (if you start in Tomorrowland, you can make a round trip without getting off at Downtown Disney).

This means that you have to have a Disneyland Park ticket to board the monorail in Downtown Disney.