i think 300-350 is average price. they are 280mm x 30mm, and i think the 280's look a tad small with 18" wheels. i have R32 GTR front brakes and they fill up the area nicely. Im not sure if its on here, but here's this too from Zilvia. Just a nice thing to have.
R32 T : 280mm x 26mm (single pot sumitomo caliper)
R32 GTS-T (early) : 280mm x 26mm (single pot sumitomo caliper)
R32 GTS-T type m : 280mm x 30mm (4 pot sumitomo caliper)
R32 GTR : 296mm x 32mm (4 pot sumitomo caliper)
R32 GTR VSPEC : 324mm x 30mm (4 pot brembo caliper)
R33 GTS-25 : 280mm x 26mm (single pot sumitomo caliper)
R33 GTS-4 : 280mm x 26mm (single pot sumitomo caliper)
R33 GTS-25T : 296mm x 30mm (4 pot sumitomo caliper)
R33 GTR : 324mm x 32mm (4 pot brembo caliper)
R33 GTR VSPEC : 324mm x 30mm (4 pot brembo caliper)
R34 GT : 296mm x 30mm (4 pot sumitomo caliper)
R34 GTT : 310mm x 30mm (4 pot sumitomo caliper)
R34 GTR : 324mm x 32mm (4 pot brembo caliper)
Z33 NA : 324mm x 32mm (4 pot brembo caliper)
Z32 NA : 280mm x 26mm (4 pot sumitomo caliper)
Z32 TT : 280mm x 30mm (4 pot ali sumitomo caliper)
S14, S15 : 280mm x 30mm (4 pot sumitomo caliper)