Two quick comments ---
1. A custom-fit suit does a very nice job of providing warmth and thermal protection with a minimum of drag, excess material, and discomfort. Many drysuit divers who went with the custom fit will continue to do so with subsequent drysuits. There is a definite difference.
2. Much of the cost can be amortized out over the life of the drysuit. I got mine in the Fall of 2002. It is now the Spring of 2010, and I'm still diving it. Thus it will soon be 8 years old, and at an initial cost of some $2200 or thereabouts (?), I've paid about $275 per year to dive it - not including neck seals, wrist seals, and other standard maintenance costs. It's expensive, but not hideously so. I've had close dive buddies tell me that I paid far too much for a custom fit, but during the same 8 year period they've purchased three cheaper off-the-rack drysuits looking for a better fitting product. Nowadays we don't discuss the matter very often, as it is apparently a sore subject.
The only caveat to getting a custom suit is that if you truly intend to dive it for 8-10 years, (depending on how old you are when you get it,) be prepared to discover that the damned thing has mysteriously shrunken a bit some 5-6 years after you buy it. And in another 3-4 years, it has mysteriously shrunken a bit more. Probably something in the water. Anyway, it may be wise to ensure your waist and chest measurements are an inch or two to the large side when you have yourself measured for the custom cut. Just a thought.....
The only other item of note is that I've used my drysuit in water that was nearly 32 degrees F up to water nearly 75 degrees F, ...you vary the undergarment accordingly. Particularly if you are doing deeper mixed gas dives, IMHO there really is no better option out there for most diving than a good drysuit. They are reliable, flexible, and offer excellent thermal protection, and a good one will tend to last for many years. Aside from cost, (and depending on what sort of diving you intend to perform,) there really is no downside to a custom fit drysuit IMHO.
YMMV.
