I take out about 8 - 10 of these policies per year to cover foreign films that I buy at my day job. In my case, I only take out policies when they are required to fulfill obligations to say, HBO or Netflix, for example and only when the revenue to be gained clearly outweighs the cost. The broker usually needs to see the specific licenses that I want the policy to cover and the cost ranges from $5k - $15k, depending on the media formats and other variables. In my case the deductibles range from $10k-25k and the coverage limits are usually either $1m(single)/$5m(aggregate) or $1m/3m.
I have no idea what the cost would be for your situation as a self-publisher, but it certainly wouldn't hurt to get a few quotes. Most likely, it would only be worthwhile if you think there is a strong likelihood someone depicted in your book will sue you and unfortunately the amount of the premium will be directly proportional to this likelihood.
If you haven't already, I would talk with an advisor about setting up an LLC to receive your web and publishing income and possibly transferring the rights in your novel to your publishing company.
