You can obtain your own quote online and see how it compares with your current quote. Just do a Google search for strata insurance brokers and complete the online application and they will reply with a quote. You can obtain most of the details from your current policy.
Generally the commission is split between the broker and the SM so in your case each get 10%.
If your SM is adding 20% on that is excessive and you could save that by dealing directly with the broker for all claims (just like a normal person does) however your SM won’t like this because he is down his 20%.