Clutch life depends on several factors: how much you ride, how hard you are on the clutch, how often you change your oil, etc...
I have owned both KX 80's and YZ 80's and think you will get longer clutch life out of the KX. And, as stated above, the KX has a smoother power delivery (because of the power valve engine) compared to the YZ. But the YZ does fly when you ride on the pipe. So if you're a beginner, I'd go for a KX. If you're more experienced, maybe the YZ. I also had a '99 KX 80 and thought it was a great bike. Never had a mechanical failure or anything break. I didn't have as good of luck with my YZ.
A new 80cc clutch (for just plates and springs) should run ya somewhere around $70-$100. If you need new baskets/pressure plate... then it can get expensive.
Check out the Mini Forum for lots of good info to help you out. If Ivan wants, he can move your post to the Mini Forum also, as you might get more responses.