It is difficult to ascertain an exact figure for each car,

We expect to gain 30% on both BHP and Torque for a turbocharged diesel engine.

For a turbocharged engine we expect increases such as:
Example:                 BMW 320D 163BHP increased to 198BHP &71lbs/ft torque
Example:                 AUDI A3 TDI 140BHP increased to 170BHP &75lbs/ft torque

It will take literally 6.5 tanks to make your money back, as you will gain upto 60 miles extra per tank (12 gallons). Equal To 5MPG increase. An engine that responds more eagerly to your right foot,smoother power delivery making town driving much easier with fewer gear changes, improved acceleration for safer overtaking