Depends. In a relatively aerodynamic car with 2.76 or numerically higher gears it doesn't get you much. The less road load and the higher the gears (numerically) the less torque it takes to move the car and therefore the converter slips less. The pay back would likely not be reached in your lifetime assuming you had to pay to have the lockup trans rebuilt. But if you do foolish things like the factory did with late 318 cars and 2.45 gears the lockup helps more as the car is heavier, less aerodynamic and has less torque multiplication in the rear axle.