No offence mate but i think $5500 for comprehensive insurance on an R33 gtr for a 21 year old is good value.
And of course they arent willing to negotiate, why would they? Your insurance premium is worked out by using a formula, that includes things like your age, your insurance history, your no claim bonus, the cost and type of car, where you live, whether the car is garaged, how many mods etc etc...