It depends on your gross receipts. I write/quote a lot of contractors, but I don't think that I've done a plumber yet. I'll ask around tomorrow for some more info on which companies I have that will write plumbers. It depends what you do as well. $1,200 sounds reasonable depending on what you do.
Also, are you talking about a $1 million umbrella in addition to a $1 million/$2 million regular general liability policy?