Given the regional differences in pricing in the painting business and the different types of painting businesses, there is no single average that would be meaningful in all places. Your liability insurance costs will vary depending on your broker, the kind of painting you do, where you work and other factors. If you are a sole proprietor, chances are that your insurance costs will be relatively low. As soon as you take on an employee, expect them to go up, and if you have a dozen employees and work in higher risk environments you will pay even more.
Get a quote from several brokers in your areas on a set amount of liability insurance for your painting business. A standard amount for a sole proprietor is $1 million. Give each broker consistent information so you can get a reliable number to compare. An average amount for annual coverage in Pennsylvania is around $500; an average in the state of Florida would be about $1,200, according to contractors.
Looking up a cost for insurance for Massachusetts will be meaningless in California. It is possible that your costs for such a policy would be in the $100 per month range, but that could escalate dramatically if you do work in museums, for example. Compare the policies carefully when you get the quotes from the broker.
Some policies do not cover power washing; others may not cover the substrate you work on, which won't be much help if you accidentally damage a wall. Comparing apples to apples is the only way to make a meaningful, informed decision for your business.
You can take many steps you can take to keep insurance costs down, beginning with always keeping safety in mind. If you do not need to file any claims, the insurance company will see you as a good customer. If you have to file for a couple of accidents, expect your rates to rise when you renew your policy. If you arrange to pay your insurance broker a flat fee to procure insurance for you, the broker won't benefit further by selling you insurance you may not need.
If you do need to file a claim, be able to demonstrate safety precautions you have implemented to keep it from happening again. Ask your broker for a range of plans based on different proportions of jobs you are doing. If residential work is much more costly to insure than commercial, you may want to consider more of the latter if your insurance costs are rising.