Believe it or not, when I added both my kids to my policy when they started driving it was right at $100 each. Both had their own vehicles which were about 10 years old... a Saturn 2-dr for the daughter and a Ford F-150 4x4 for my so.
I just dropped my son off our policy at age 22. My daughter is still on our policy.
Up until recently we had 2010 F-150 full coverage (me), 2013 Scion TC full coverage (son), 2010 Acura TL full coverage (wife), and a 2013 Kia Sorrento (daughter) full coverage was all under $400.00 a month IIRC. High deductible though at $1000 each.
My son just got his own policy at age 23, 2014 Jeep Wrangler Unlimited, full coverage, about $80.00 a month.
Full disclosure... I've been with Geico since 1986 with zero claims and zero citations across the board.
ETA- more I think about it I believe it was under $350.00. Just looked it up, $356.