Not very, since it's illegal and the rangers confiscate your parachute if they catch you.
I can think of two deaths of base jumpers. One guy drowned when tried to swim across the Merced River. He took off when he saw that the rangers had seen him.
Another was a woman who jumped with a spare chute, she didn't want to lose her main chute when it was confiscated. Her shoulder got dislocated when she jumped. She wasn't familiar with the spare chute and wasn't able to get to the main or spare release with her other hand in time.
These were both off of El Capitan, not Half Dome.
The only NPS sanctioned activity that involves jumping off of a cliff is Para/Hang gliding off of Glacier Point, and it is VERY tightly controlled.
The NPS has a working relationship with the hang gliding community.
NPS has always had a very cantankerous relationship with the base jumping community.