I’ve had it for 31 years and have never had to use it. Only my contracts with government agencies have required it.
When I started business, I went to a local broker and showed him contracts that required it. He said as a graphic designer I would probably never have an issue where professional liability came up, but there were circumstances where I could be exposed. He suggested the amount of liability they were requiring me to have was very high, considering my profession and the nature of the work I was doing for them, and that I should ask if they could lower it. He also suggested they waive their requirement on auto insurance, since it was also high, and I wouldn’t be doing any driving on their behalf. He said a lot of times people are just throwing boilerplate at you, and they don’t even understand it, and it’s irrelevant to the actual risks of the task you are performing, so negotiate.
Mine is $560/year. Currently through that same broker, and it’s with Liberty Mutual, but it changes every couple years, and not because I requested it. I get the idea that the policies are sold between these companies. In the past it’s also been with Safeco, Golden Eagle, and some others.