As I'm sure you're already aware Jamie - you're looking at 15-16k at a dealer for a 2008 roadster with 30k on the clock - so really you should be able to aim for similar to that considering your mileage and mint condition - just depends if you're prepared to ride it out.
Not many will have this sort of cash though so as Liam says get it up on PH and AT and see if you get some more interest.
I do feel though that the comments about early 370z going for 3-4k more are unhelpful folks, you certainly wouldn't get a roadster at that sort of price and they are two completely different beasts