That’s what I would do too, but I haven’t found a desktop build that can rival a Mac Mini or Mac Studio on performance per watt. I haven’t looked super hard, but it seems like Mac is in a different ballpark.
I mean, once it's a desktop, watts are pretty cheap, so it's a bit strange to optimize on that factor for the desktop form factor. For laptops it makes a ton of sense.
If I was optimizing solely for cost I wouldn't use a desktop in the first place. Watts are cheap, but they result in pollution and noise (e.g., fan) or you need some costly alternative. Also I just value efficiency.