Seems like we're all (willing) victims of the syndrome. I myself am poor, so that tends to create a practical ceiling to my spending, but even so I've spent more than I intended or imagined I would on many a build. When I started this hobby I'd keep track of spending in my head or on hastily written notes (post-its, napkins), but after a while I got organized and started keeping a running excel spreadsheet for each project. As soon as I buy a bike or a frame, or oftentimes even beforehand, I'll forecast all my expected costs. Very often that'll force me to re-evaluate the direction of the project and bring my plans down to earth.
The other nice thing about this is that now I have a record of expenses and revenue stretching back for over a year (20 or 30 bikes' worth). Looking back over those spreadsheets is instructive. One thing I can tell for sure, although I guess I already suspected as much, is that the more time and money I invest in a project, the less likely I am to turn a profit or even break even.