VBAC?

I have a question for VBAC mommies.
Why are they considered higher risk? Is it from the scaring in uterus? What makes them cost more according to hospital? What makes them so dangerous?
I DO NOT MEAN THIS IN A RUDE WAY!! I AM CURIOUS! I am a FTM and have never had either. I just was reading over papers from the doctor and they said they carry higher risks than any other delivery and cost about a grand more than any other way.
Thank you so much!!!