Back in 02-04 I worked at a prominent DSM shop outside of D.C. It was absolutely childs play to get to 450-500hp in a DSM without even lifting the head off the block. Problem is at that level you are tuned pretty close to the edge. One tank of sub par pump gas and you are toast. Good example:
Personal GSX stock longblock, t3/t4 setup on a 60-1 compressor side, AEM EMS, 720 injectors put down 469hp to the wheels without pushing it too much. I ran like that for almost a year on pump gas, but one night I must have had a bad tank... Hit a little detonation and the #1 piston decided to come apart, demolishing the shortblock, head, and turbo.
On the 4g63 the stock cast pistons are the weak link, a little knock and poof you are gone. We normally suggested our customers to go with rods/pistons at the 450hp mark, and if not turn down the boost on pump, and only run full boost on c16.
Even though I didn't do a LOT to my car to get to 469whp, I did end up spending around $7k, and did all the work myself and was getting dirt cheap prices cause I worked at the shop.
EMS: $1400
Turb: $800
Exhaust: $800
t3/t4 manifold: $300
Wastegate: $200
Injectors/Pump: $500
Boost Controler: $400
FMIC: $1200
Clutch/Flywheel: $600
Misc: $???
So although it was easy to get there...staying there in one piece without a little internal engine work was a little more difficult
Heck, my cost on rods/pistons was only $800, and could have gotten the machine work for another $200. For another $1000 I could have made that setup a completely reliable venture. Again, doing all the work myself.
If we were to do the install on all that the bill would have been around $3500....
So this 370z kit is VERY reasonable.