Oh wait, there is also a "Category:Biography articles with listas parameter" with 140,000 members. If the with/without "listas" are assumed to be a complete set then there are 445,000 biography articles, which would give roughly equal numbers of living and dead biographies. So ~22% of Wikipedia is biographies.
Sounds about right to me.
Conclusive proof: I just clicked Random Page 10 times and got 2 biographies (didn't check if they were alive or not).