Chris Boyce of the University of Warwick and Alex Wood of the University of Manchester compared large data sets where 1000s of people had reported on their well-being. They then looked at how well-being changed due to therapy compared to getting sudden increases in income, such as through lottery wins or pay rises.
They found that a 4 month course of psychological therapy had a large effect on well-being. They then showed that the increase in well-being from an $1300 course of therapy was so large that it would take a pay rise of over $40,000 to achieve an equivalent increase in well-being.