This was the student pilot's last long solo cross country towards attaining his private pilot certificate. According to the pilot, he had completed the preflight planning, and his instructor approved the route of flight. The instructor also advised the student that he should, 'top off the tanks.' The pilot reported that due to not having enough money to fill the tanks he added 5 gallons of fuel, which he considered was adequate for his return flight. The pilot stated that on his return flight, he made some additional landings en route that were not calculated previously. During cruise flight at about 2500 feet and 15 miles south of the airport, the engine lost power. The pilot stated that he tried to restart the engine, but he was unsuccessful. A forced landing was made in a marshy field and the airplane nosed over. Examination of the airplane revealed a total of about 1 pint of fuel in both tanks. The pilot stated that the accident could have been prevented if he had added more fuel.