I recently talked to a friend of mine that sells business insurance and he recommended at least a $1M coverage. He actually said he gets uncomfortable whenever someone asks for less than that in today's market.
Anyway, he said quotes can range anywhere from $400 to $1500 depending on a number of factors. One of the biggest factors is how long you've been an established business. If you just started you are more likely to pay a higher premium or even be denied coverage.
I recommend finding a small independent agent near you, talk with them a little and let them do the work of finding an insurance company that will take the account.