There are a few reasons that someone needs to go custom versus mass produced: They have physical issues that cannot be addressed without a custom build; they want a feature or features that are not readily available in mass produced bikes, e.g., S & S couplers for travel; or they want a specific frame material that isn't readily available in the mass market, e.g, steel and ti. There are probably additional reasons, but those are what come to my mind.

Otherwise, it's largely a question of want. Now, I take no issue with those wants. I've drooled over way too many custom bikes to ever criticize somebody for going that route when they could otherwise buy an off-the- rack bike. Some legitimately want something unique, and that's a valid reason in my book. But I think it's important to appreciate that there is a risk with a custom bike that it won't end up being the bike of one's dream bike. I can attest to that, and I know of a few others on TE who were ultimately unhappy with a custom bike. It can be a costly mistake, just as any bike purchase can be. Any time you can test ride a bike, especially if you can go on a long test ride, before buying you are at an advantage IMO.

I don't know if that helps you navigate what you want. I, personally, might be very happy with a carbon bike that was well short of top of the line, so I think there may be some benefit to figuring out what you really need/want in a bike--not what your budget can theoretically allow. In other words, don't just spend a princely sum because you can. Figure out if the various upgrades make sense for how and where you ride. If you do that wisely, maybe your pantry won't have to put off for another year!

Also, in running your numbers, I wasn't clear whether you were also factoring in wheels in terms of the cost of custom. That can add another $600+ dollars (or well more) pretty easily.