County officials say the bulk of the costs are to repair roads and bridges that were washed away. The Coachella Valley sustained the brunt of the damage.
National Weather Service totals show Hilary dumped some three inches of rain there.
“When water is also accompanied by mud and debris flow, that means there's trees, there's rocks, there's boulders, it makes cleanup even more difficult," said Brooke Federico, a public information officer at Riverside County.
Federico says the cost of recovery could grow as damage assessment continues.
Climate change is fueling more extreme weather events that make recovery more expensive.