According to this article, estimates of the total cost of the Soviet Union's support to the North Vietnam government range from $3.6 billion to $8 billion.
China too lent its support
The Chinese government, under the administration of Mao Zedong, took an active role in the First Indochina War. In April 1950, Viet Minh formally requested military aid including equipment, advisors, and training for the People's Army of Vietnam (PAVN). The PRC began to send their advisors and later form the Chinese Military Advisory Group (CMAG) to assist the North Vietnamese forces in return, led by General Wei Guoqing, along with Senior General Chen Geng. This is the beginning of China’s assistance.
Seeing as how the Cold War was more of a clash of Ideology between two world powers, arms deals made or military support given would not have had the primary objective of profit or commercial gain. After the Vietnam war, aside from an ideological and military victory over the west; did the Soviets and China (for their support during the IndoChina war as well) expect anything in return monetarily?