Florida's GDP is $748B. Suppose 1 in 300 people in the state are mentally ill enough at a given time to warrant living in a mental hospital. If that costs $1B, about half as much of the economy would be spent on them (per capita), compared to the general population. Seems very reasonable to me, without any more specific numbers to go on.
To throw out another number for comparison, medicare and medicaid together cost about $50B / year in the state.