The University and College Union research indicated universities spent thousands more on hotels, flights and other expenses for vice-chancellors.
The UCU says staff pay is held down but bosses get "inflation-busting" rises.
Universities UK said the pay of bosses was in line with similarly sized public and private organisations.
The UCU used Freedom of Information requests to ask UK universities and colleges questions about vice-chancellors' pay deals, including benefits and pensions, for the year 2014-15.
The union also asked about money paid out for air fares, hotel accommodation and personal expenses.
The findings suggest the average overall salary package for university heads in 2014-15 was £272,432  (based on figures from 152 institutions) - up from £260,290 in 2013-14  (based on figures from 150 institutions).
The report also suggests:
Eighteen of the 159 institutions contacted did not respond to the Freedom of Information request.
A Universities UK representative said: "The remuneration packages of vice-chancellors are determined by independent remuneration committees at each individual institution and are publicly available in universities' annual reports and accounts.
"The salaries of university leaders in the UK are in line with those in competitor countries and comparable to similarly sized public and private organisations."
A spokesman for Oxford University said the institution was consistently ranked as one of the best in the world.
"Its research output is vast, it has more than £1bn a year in turnover, not including the colleges and Oxford University Press, and it has great institutional complexity," he said.
"The vice-chancellor's salary reflects that."
A spokesman for Glasgow Caledonian University said Prof Gillies "maximises the value of each business trip by fulfilling a full schedule of engagements".
"GCU is transparent in reporting costs incurred, and Prof Gillies's travel and accommodation costs are subject to scrutiny by independent governors on the university court."
A University of Strathclyde spokesman said: "The principal is a global ambassador for the university, and forging links with international institutions, academic partners and industrial collaborators is an essential part of his role."
But UCU general secretary Sally Hunt said: "The time has finally come for a frank and open discussion about pay and transparency in higher education.
"The huge disparities in the levels of pay and pay rises at the top expose the arbitrary nature of senior pay in our universities.
"While some continue to enjoy inflation-busting pay hikes and all the trimmings of first-class flights and luxury hotels, staff pay continues to be held down.
"We will continue to campaign for a proper register of pay and perks at the top of our universities."
Vice-chancellors at UK universities received average salary packages of £272,000 last year - up by some £12,000 on the previous year, a study suggests.