What is the Average cost of general liability insurance for sole proprietor businesses?

It is not possible to state an "average" cost, as too many variables are involved. For example, the cost of GL coverage will vary by the nature of the business that you are in, where (geographically) you are located, your loss history, by the insurer that issues the quote, and by the policy limits sought. In the latter respect, there may be a difference in the quote between policies that have separate property damage and bodily injury limits vs. a policy with a combined single limit. The deductible or self-insured retention that you select will also have an impact.

In all events, it is critical that you deal only with an insurer that is authorized (licensed) to transact insurance in your State. It is only those that have the backing of the state's guaranty fund in the event that the company is unable to honor its claim from its own resources and reinsurance. It is also important that you deal with a reputable and licensed agent or broker who is able to take your full financial picture into account.