HART and the city formed a group to study bus/rail fares a while ago.  (It's planned to be one set of fares.)  The city already has experience with bus fares and the subsidy.  They have projected ridership for rail.  They've given us a rate of 40%.  Either they're pulling that number out of the air, or they have some idea of what the fares will be.  

In addition, Honolulu currently doesn't have zone fares or fares based on distance, so that part is easier than some other places.