A small plumber typically pays from about $780/year for general liability insurance — the exact figure depends on revenue, payroll and location.
Insurers price on your trade's risk class plus revenue. A plumber sits at roughly 2.6x the base office rate. Estimate yours in the GL cost calculator.
Add workers' comp if you have employees (calculator), and consider a BOP to bundle property. See the full picture in the total cost calculator.