Use a text editor such as notepad to edit the "missionbrief.txt and "missioncompleted1.txt" files in the "/data/missions//" folder. In the "missionbrief.txt" file, look for the amount of pesos and increase it to the number desired. In the "missioncompleted1.txt" file, look for the amount of pesos and set the same number. Play the mission and get your reward.