Should have been a bit more clear I guess.

IMO if you start with a used frame, and only use used parts it's still going to come out more expensive than just buying a similar complete used bike. And of course there is always the temptation to buy new stuff.

When talking about new, it's always more expensive. I'm not talking about buliding something half used half new that's "comparable" to a new bike, just as an example try to build up the exact same thing. Pick a bike that's available as a complete and just as a frameset, and from the frameset build the exact same bike as the complete bike using the best prices you can find online. It's going to be more expensive, no question. (Or just pick a complete groupset and add up the components separately, same thing.) Yeah, dumb example, but you get the same result if you build a custom bike from all new and up to date stuff.

Of course building is fun and I just did it with 100% brand new parts including a frameset, a complete financial disaster.

When the main objective is maximum bang for buck I still think a complete used bike wins.