If Pi Clusters were actually cost competitive for performance there would be data centres full of them.
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If one just wants a cheap desktop box to do desktop things with, then they're a terrible option, price-wise, compared to things like used corpo mini-PCs.
But they're reasonably cost-competitive with other new (not used!) small computers that are tinkerer-friendly, and unlike many similar constructs there's a plethora of community-driven support for doing useful things with the unusual interfaces they expose.