Dr Johnston, whose own institution is renowned for its access policies, said there were still many more young people who could benefit from a university education. "Educated people are more civilised, more culturally aware, more independent and contribute more to the intellectual and political debate," he said.
Sunrise jobs of the future would depend on educating young people and offering adults a second chance.
Scotland already had 47 per cent of its 18-year-olds in higher education, in contrast to around 30 per cent in England, but more could be done to attract students from low-income families. It was not a reflection of their ability that many did not go.
Dr Johnston said: "We've got quite a lot to do both in improving secondary education in some of the most deprived areas of the country and in removing any sense of class bias that might still remain in the system."
Secondary education, he added, was improving and more young people were qualifying each year for university entry.
But from a university viewpoint, it cost more to educate students who came with "a cultural deficit". Dr Johnston said their parents would often not have read books or owned computers. Their offspring needed educational and practical support. "There is a cost in socia inclusion which is not fully recognised at the present time," he said.
Restoring appropriate funding to the higher education sector would cost pound;50 million above revised Scottish Executive spending plans, he suggested.
Fifteen years ago, universities received pound;100 per student, against the pound;60 they now receive. "That's a huge efficiency gain by any standard," he said.
Dr Johnston called for a 10 per cent increase in lecturers' pay, particularly important at the top and bottom ends. "The key issue is to attract active, young, bright academics into the profession and there's a real risk of the brain drain," he added.
Breaking down the figures, Dr Johnston predicted it would cost up to pound;50m to improve teaching and between pound;30m and pound;40m for pay. The Executive last week revealed the higher education budget would rise over three years from pound;609.4m to pound;697.6m, an pound;88m increase. But this still left a deficit, the principal said.
Bill Stewart, the president of the Association of University Teachers (Scotland), said that since cash per student had fallen by a third in real terms over the past 15 years, it would cost up to pound;180 extra to restore funding levels.
Dr Johnston believed that figure was unrealistic. It was also right the public should expect greater efficiency in universities, but cuts were now "down to the bone".
John Swinney, new SNP leader, labelled the figures "a dose of reality". He said: "University lecturers are being asked to do more for less with more people. That's an enormous burden."