The deal was simple: rent for 5 years = $100m dollars (made up numbers)
Buy outright = $150m dollars. Therefore 'cheaper' to lease them.
However, after the hours were overflown, and the lease was extended, they cost $450m dollars.
Like all the other PFIs and contracts of that era, a short term fix that saddles the bill payer with higher costs later down the line, presumably on another government/chancellors watch!