They are forced to rely on the sale of membership cards and commercial activities such as shops and games machines to survive, according to the research by the National Union of Students.
The block grant averages pound;5,000 a year but is variable. One in five colleges gives no block grant, leaving the union dependent on fund-raising.
Of those that contribute, the lowest block grant comes from East Surrey, which gives pound;1,000 a year; the highest, Derby College, pays Pounds 165,000.
The generosity of colleges bears virtually no relation to the number of students enrolled. The average student union grant is just 0.05 per cent of the college budget.
Publication of an interim report of the survey this week coincided with a lobby of parliament by college employers, unions and students for an increase in government spending on FE.
Sian Davies, head of the NUS campaign, said there was clear evidence that student unions were suffering as a result of the underfunding of colleges.
She said: "Many students cannot participate in clubs or societies, an essential ingredient of student life, and volunteering opportunities are not available in this, the Year of the Volunteer.
"This survey clearly shows that the FE experience is suffering as a result of massive underfunding. Furthermore, basic support levels are just not available as student officers are overworked."
Average college revenue from NUS cards is pound;2,017, while pound;400 to pound;8,000 is raised through shops and gaming machines.
Most student unions get the support of a staff-student liaison officer, but these are rarely full-time and most have other duties.
John Brennan, chief executive of the Association of Colleges, speaking at the lobby, said the lack of support was an indication of just how much underfunding was affecting colleges.
Colleges had asked the Government for pound;1.9 billion extra funding between 2005 and 2008.