To get a "decent" R33 GTS-t ur gonna need $20,000 and to get into the very very high 13's your going to need ~$5,000 in mods (thats if the car is 100% stock to begin with).
Their are some R33's out there doing mid 13's on a very small budget but these guys can really "drive" the thing. Most people just need a fatter wallet to achieve the same outcome.
Most people will need:
* Full exhaust
* Pod Filter
* AFtermarket ECU
* Good Rear Tyres
* Aftermarket Rear Suspension
* Boost
..and thats already ~$4,000.
Hell i've pretty much got the above mods (minus ECU) and im still playing around in the 14.1's
Edit: Take a look at the "SAU 1/4 Mile List" on my website (http://skylines.pnc.com.au) and you might want to PM some of the guys to see what mods they have to get the results they got.