Ordering doors and drawers from the many suppliers of the same can be pretty economical when you account for both time and tooling. The single downside to ordering doors is that they may not be quite as particular relative to grain matching that some folks (like me) prefer for stain grade work, but that's pretty much consistent with a large percentage of orderable cabinetry anyway unless you buy full custom for big bucks and specify accordingly. And for paint grade...it's a no-brainer, honestly, especially for a full kitchen. Build the boxes for sure. But there's no harm in considering buying the doors and drawers (the latter preferable wherever they can be used in a kitchen, IMHO) and put your time into other details. You don't necessarily save any money building your own, honestly. It's mostly just the "satisfaction" factor...
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The most expensive tool is the one you buy "cheaply" and often...