Oh right i see. So eliminating rent (and maybe food?) you'll still want some cash for books / course costs as well as your recreational activities (you might end up drinking a lot in your first year - drinks are expensive!). Just something to bear in mind.
As for your initial Q, have you tried putting more adults on your policy? I'm not sure how it works with m/c insurance but with my car insurance I put my dad down as an additional driver, it lowered the price by several hundred quid.
Also, I wouldn't say your quote is out of the realm of possibility, it is high, sure, but i can believe that. When i upgraded my motor to a 1.6L at 22 (2 years NCB) i was getting quotes of around £2.5k for comprehensive cover. Again, the biggest drop will be as you get older - but in the meantime play around with comparison sites and your details i.e. change from garaged to drive or to on street, add additional drivers, etc. You'd be surprised - sometimes its cheaper to list your vehicle as on street parked as opposed to garaged.