In our case the cost was $0, but you are correct that the interest rate formula (prime + X) is higher than others which were available at the time. This served our purpose, because we expect not to use the line, but the interest rate should be a consideration if you are intending to use it.
I spoke with our agent (salesman?) the other day, and our out of pocket cost would also be $0 if we don’t use it and keep it open at least 3 years. Seems like there are a few options out there that do them for free. I’m leaning toward a few more accounts with local CUs for deposits though.