The total runtime for the first three machines is 3 machines * 23 hours/day/machine = <<3*23=69>>69 hours/day.
So these machines could produce 69 hours/day * 2 kg/hour = <<69*2=138>>138 kg/day.
The fourth machine could produce 12 hours/day * 2 kg/hour = <<12*2=24>>24 kg/day of material.
In total all the machines would produce 138 kg/day + 24 kg/day = <<138+24=162>>162 kg/day of material.
The factory would therefore earn 162 kg/day * $50/kg = $<<162*50=8100>>8100/day.
#### 8100