How expensive can it be? It costs me about $1 a day per vehicle (for
liability) with a 100K/300K coverage with a $500 deductible. I used other
cost saving measures but I'm sure other people pay about the same.

The reason they still charge you is because they still have to cover the
possibility that you allow someone without insurance to drive one of your
vehicles whilst you drive another one of your other covered vehicles.

BenT
San Francisco