Russell,
Yes, you must have a liability insurance policy to cover drivers. Even if they own their own cars. Of course, all markets are different with pricing so my suggestion to you would be to contact 3 insurance agents in your area. Let all 3 know you’re trying to get a quote and you can get you an exact price on what it would cost to insure drivers in your area.
If you’re looking for a ballpark: some insurance companies structure things a little differently, but you shouldn’t pay more than $800 a month to be WELL covered (drivers, $2m umbrella, flood, fire, etc.). The average cost per month, however, should be in the $400-$600 range. Hope this helps. -J_r0kk