If it rejected 99.9% of patents, the expected value of the typical application drops to 1/1000 its current value. So fewer people would pay the application fee.

If it rejected 99.9% of patents, the expected value of the typical application drops to 1/1000 its current value. So fewer people would pay the application fee. Fewer patent clerks would be needed, so their operating costs would also decrease. But presumably not below 1/1000.