Don't think its the individual piece parts per-se. I think its a continuum of:
1) The granularity of the strip process of the donor car (i.e. do you strip to unibody and replace every fastener coming back up? I did this with my e30, a non-trivial exercise...)
2) Replace/rework existing systems on the chassis. the only part of the oem fuel system on e30 is the tank, *everything* else is different. The brake system above the rotors is 100% different from OEM. It has coilovers with camber plates (to include the condor e30 specific rear adjusters for the TA's). Every bushing on the car is Delrin, and the guibo is the revshift one. I mean I hit everything.
3) Electrical: I threw everything away, and I mean everything. It has a motec PDM and AEM ECU, with multiple tunes. There were drivers for this: reliability and weight, in that order.
4) Motor. some can be made better reasonably, some can't. the first pass of mods i did on the M20 probably bought another ~15-20hp. meh. The next pass made it insane (but this was pricey, there's easily 10k in that motor at this point).
5) Optimization. You can make CF body panels very easily. Room temp pre-preg is cheap and the process is *dirt simple*. (I think the tape thing is ridiculous if that's actually what the boxster people are doing for body panels.). The panels is next on the list for the e30.
But even after all of that, there's prob at least another 18-24 months to fully develop the car as we are just now diving in for real after a year of it basically sitting (which is a criminal offense). This is where I agree with @Grant on key dimension: setup.
If you do all these things I mentioned above in list, then the investment is high. I'm into the e30 for easily 50k at this point and I think when you look across the spectrum of CC teams there are lots of them at this investment level on their build, some winning, some not.
When I started the e30, I had a vision / idea of what it meant to build a race car. That vision/idea was really at odds with the CC rules. I went back and forth on whether or not to commit to the rule or just build what I really wanted to build. The deciding factor was when I was building the fuel system and CC changed the book like twice in a short space of time which flushed a bunch of hours of effort on my part in the garage. That was also a bad flashback on past life experiences with rules systems in another sport and I just bailed on CC and built what I wanted to at that point, reorienting on WRL. That event for me seems apropos other threads on BCCR, so to close out:
I've opined on the CC rules for a while. Straight-up I don't like them, too nit-picky. I think they are trying to please too many masters and when you have dealt with handicapping systems in competition you know none are perfect, and this one is now distorted in a really bad way (which ends badly). Reasonable people can disagree on the structure of handicapping systems, but one thing is for sure, you will never please everyone. The best you can do is to very, very clearly define the strategic objectives of the rule system and structure the rule-set appropriately. I think this is where the gap is currently in CC, at least i.m.h.o.