Clutches - It depends on the car. Clutches for some cars are very spendy - I was hit for £800 for my TDI although that was an uprated one including a new SMF flywheel with a view to performance improvements (not done, but at least it won't wear out
). That was for parts alone, and a standard one including the DMF is not much less.
DMFs are not always replaced but sometimes they are on the basis of "whilst we have it in pieces may as well do that as well" - like renewing the TDI water pump when doing a belt change. Some minicabs (Taxis using normal cars) local to me have them replaced when new with SMFs.
On the other hand the clutch for a Toyota Aygo costs only around £170 fitted. I'd be tempted to track day or stage rally one every weekend at that price