Going after 100% impressions share will likely be expensive. Even if Google’s Keyword Planner said CPCs were $1. That might only be true up to 95% impression share. Then to get the remaining 5%, you might need to pay $1.50 or $2 CPCs. For brand campaigns, CPCs can often double or triple as you get closer to 100% impression share.
There is no easy math on this since the ad auction is dynamic. If you are already running a brand campaign, look at what your impression share is now and how much is lost. What are you spending to get the current impression share and do some forecasting to guess what you might need to spend to get the rest. If you are not spending on a brand campaign right now, pretty hard task to do.