There are hidden prisoners on each stage
If you capture all of the prisoners (usually around 10), you'll get 100k bonus at the end of the stage. You also get another 10k if you have the tank at the end of the stage
If you die, you lose all of your prisoners.
After work Ill update with locations of the prisoners (cant remember myself), but for the first stage they are just after you drive up the waterfall/creek thing (right before the boss), shoot the rocks above and you'll see your shots hitting something. There are 4 prisoners up there