As aforementioned affordability is subjective but it depends mainly on your age and NCB in conjunction.
I'm 21 w/ 2yr NCB on one policy and 0 years on the second (I pour all my money into my cars, they're a hobby and a passion).
A 2.5L Turbo w/ a couple of modifications costs me £1300 with 2 years NCB. A 2.0L standard road car costs me £1000 with no NCB. The NCB is massive in deciding your premium - next year I should be looking at £700 for the 2.0L (hopefully with a further small discount for being a year older) and £1350 (only a quote currently) for kitting out my 2.5L Turbo up to 320bhp. Point being NCB makes a HUGE difference, so get a cheap as chips car and save for a couple of years then buy something awesome - took me four years of saving to buy my sports car and afford the insurance but was well worth it.
Where you live also makes a difference (not that you can help this factor) as does [extra] security features (mine has an immobiliser, alarm and if the ignition is activated without a key it shuts the ECU off). 21 with a year or two under your belt is when you can start owning nicer cars as far as I can tell.