Common DCT failure rate is extremely low since its first debut on the M3. There is not enough data to gather.
By the way, the cost of repairing DCT has gone down significantly over the years due to the fact that more models are using it now. Eventually, maybe more models (like 335iS, maybe 550iS, or 650iS?) are going to switch to DCT because of better fuel efficiency (mandate) and lighter weight.
I heard the first DCT failure cost $15,000 plus labor to replace. The last time I heard it's $9,500 plus labor to replace. Supply and demand dictate the cost of repair. Plus, if you're only talking about changing the clutch, it's a lot cheaper than replacing the entire unit.
As long as you don't abuse it everyday, following proper procedure BMW states that the DCT is good for the life of the car, not life of the warranty.