I bought a 4 door GTS4 with an RB26 so I don't know if I qualify as a GTR owner .
Anyway I spent 12000 getting it here + 1000 for compliance (this is of course with the now defunct 15 year rule)
3000 fixing ball joints, brake master cylinder, new clutch and a few minor things (like lights and what not)
There is some body work that needs to be done (some chips of paint which might just indicate something worse.. fingers crossed). It definately had a knock on the front which wasn't mentioned when i bought the car (and might have been added after for all I know ).
Finally there was rego, insurance, alarm, dyno (yay ben) + another 3000 odd.
It does add up but you can get really lucky too . I got a Carbon bonnet and a GTS4 (was advertised as GTS-t heh), engine is 100% perfect (leakdown and compression), nismo LSD is fine, suspension is all good, got a really nice blitz boost controller. Petrol is a biaatch.