What I pay for garage insurance will be entirely different from you most likely. Depends on what you are doing to generate revenue, gross income, area, number of employees, amount (if any) of customer traffic, blah blah.
I don't see you getting an accurate number (or even near reality) without a huge number of factors being figured into the mix. This is exactly why you can't get an insurer to give you a real number. Depending on the type of business you plan to operate, there may be trade associations that could help with your questions. Bill