Adding driver sports car insurance, average cost?

if you add another driver to your insurance like your brother, or sister or son, who is a modern driver, and your insurance costs $350 for six months. how much do you think would be additional cost to this insurance? if you can share ur experience it would be nice. i know it adjectives depends on the history of the other driver, but say the other driver did not have any accident or anything. i need to get a nonspecific impression...

Thanks in credit.
"> Your insurance will likely increase by double. I have 6 years no claims and my girlfriend basically passed so I added her. it was another 80% on the insurance as your policy goes on the unbeatable risk driver. This assumes you are a no claims driver 25+ to start with.
Cheers
It depends on the person's driving record and age. Lets say aloud they have a clean register and are older then you, consequently your insurance could possibly go down.

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