Pantasy, Cobi, Funwhole, megabloks, etc. There's tons of alt bricks at nearly any reasonable price point. But you're still not going to find these sorts of multi-thousand piece sets for <$100 because it takes at least a few cents to produce a dimensionally accurate, quality controlled bit of plastic.
But thats because they limit production to control prices, producing mass volume accurate plastic was never the bottleneck.
These are remarkably close to the raw cost of materials and tooling. The Sagrada Familia model is a 12k piece set for $800, ~$0.07/piece. Phantasy has an unlicensed Opera set (85019) [0] with 3.5k pieces for $190, $0.05/piece. That's straight up cheap. They're not limiting production to control prices. High quality injection molding (in multiple colors no less!) is hard. 2 cents over 12k pieces does add up to a nice bit of change for Lego though.
[0] https://pantasy.com/products/pantasy-the-opera-85019