The reward was boosted during a meeting called in the town of Davie, which attracted more than 100 people to launch a "barn watch" scheme.
Police believe the animals have been slaughtered for their meat, most likely sold on the black market.
However, authorities warn that the meat may not be safe, given the common use of wormers and other chemicals in horses.
Seventeen of the animals have been killed in Miami-Dade County and two in Miramar County.
Rewards currently on offer over the killings include $US5000 from the Humane Society of the United States, $US3000 from Miami Crime Stoppers and $US1000 from Broward Crime Stoppers. Chris Chiari, a Davie resident running for a seat in the Florida House, has also contributed $2000.
Local media report that further reward money was forthcoming at the meeting, with Davie resident and developer Ron Bergeron adding a $US10,000 reward of his own. Dr Robert Oller reportedly added another $2000, and another resident pledged $US100.