How much extra should it cost to add rental coverage to homeowners coverage?

Answer 1

Actually you can't just add it. For rented property you need Dwelling coverage.

Your homeowners insurance policy becomes automatically null and void when the property is rented out to another. You can have your agent endorse your policy for rental dwelling coverage or you can have your agent re-write the policy on the appropriate dwelling coverage form.

Homeowners insurance is for owner occupied homes. Dwelling insurance is the landlords insurance for rented properties