When the stadium is full, the total fees charged is 2000*$20 = $<<2000*20=40000>>40000.
When the stadium is 3/4 full, the total number of people in is 3/4*2000 = <<3/4*2000=1500>>1500 people.
The total entry fees collected when the stadium is 3/4 full is 1500*$20 = $<<1500*20=30000>>30000.
The difference in the total entry fees when the stadium is full and when it's 3/4 full is $40000-$30000 = $<<40000-30000=10000>>10000.
#### 10000