If you're talking about replacing pieces, yes, but that is true for most imports (non-honda). The problem with both 240sx's and 300zx's is that they are not abundant in scrap yards like some other Nissan cars (sentra's, maxima's, etc) as they were not a very mainstream car. Not being in junkyards as often means you'll have to buy a new part, ebay, or use car-parts.com to find a replacement for whatever needs fixing. Usually at junkyards you can cut a deal with the guy and get a fairly used part as a killer price, this is something I very rarely have been able to do, but my friends who have Rustangs, vettes, F150's, they always take advantage of junkyards and make me utterly jealous of what they can obtain. Nissan Hardbody pickups can be found in junkyards and sometimes Nissan Frontiers, so you may be able to salvage some parts from those, as they share the same motor as a stock 240sx.
If you're worried about cost, perhaps you should think about getting a domestic vehicle. Owning an import is rewarding, but its also a little taxing on the checkbook (especially when you start to mod). My buddy who has a 5.0 Rustang can do SO MUCH MORE with a grand than I could... When I told him that my exhaust system up to my turbo extention cost me about $875, he laughed at me for about half the day.