Jarvis is an insurance AGENCY. They can sell coverage from various companies. Markel is an insurance COMPANY that direct writes to consumers as well as sells via agents. Hallmark also is an insurance COMPANY. I've never used them and don't know if they direct write or not. For some reason I think they go through agents.
The cost varies depending on liability limits and activities. Last time I looked, the cost was something like $500 - $600 a year.
I am in New York which makes insurance more expensive (le sigh), and my instructor's insurance (my horses and other people's), trainer's insurance (for riding other people's horses) and care custody and control for trailering is right around $850 a year.