Without knowing what profession you are in it is impossible to know what your cost or what the average cost would be. The premium rate would be greatly affected by the type of professional exposures you have and the annual risk volume.
For example in regards to volume, a Surgeon who does 200 surgical procedures a year might pay 100 times more for his insurance premium than another surgeon who does only 20 or 30 of the same procedures per year.
It's not just your profession but also the type of services you provide, the risks associated with those services and the number of times you provide them in a given policy term. This is what the insurance company refers to as "risk exposure".
As you can see someone who does 100 times more operations would naturally have 100 times higher potential to make a mistake that could generate a claim or a law suit.
There is no average cost. Each underwriter has it's own underwriting guidelines and risk assessment factors.