Georgie
Nobody will be able to give you a rough idea on his forum. It will depend on which Focus model, your postcode and alot of other information. My rough idea would be that it will probably cost you more than the cars are worth.
Would suggest that you do a google search for young drivers Insurance. Endsleigh have a 17to40 site that can offer quotes. Give it a try and phone others.
I started off with an old fiesta 1.1 for a couple of years. The money I saved on Insurance in the those years, enabled me to buy a much better car a few years later.
huckster