Another big factor in pricing, which I don’t think I saw mentioned is whether the umbrella covers uninsured and underinsured motorist coverage. I do remember it was pretty expensive to add UM/UIM when I got my quote. This will vary greatly by state as state laws are all over the place regarding UM/UIM coverage.
At least in my state, if you're injured by uninsured/underinsured motorist, your own umbrella policy wouldn't pay anything. That's what UM/UIM coverage is for.