Usually the DMF doesn't cause a clutch failure, but replacing the clutch without the flywheel is a no-no...
A DMF is basically a flywheel made of 3 parts. An inner disc which bolts to the crank; a outer ring which has the teeth for the starter and the friction surface for the clutch plate; and a rubber filler which joins the two taking out small vibrations from standard clutch operation.
How expencive? About £400-£450 for the flywheel making a + labour clutch change around £800-£1300 depending on how difficult the job is. 1.9JTD should be closer to £800, some of us ex 2.4JTD owners paid the bigger number!