"Y-Combinator’s success is undeniable: the program has produced 37 start-ups worth over $40 million each in the past five years. In its first six years, 72 percent of Y.C.’s 249 start-ups raised money after Demo Day, and today, the average value of a Y.C.-financed start-up is $22.4 million. However, these statistics also reveal a grimmer reality: 93 percent of the 511 companies accepted by Y-Combinator have failed. Even more alarming, only 3 to 5 percent of the companies that apply to Y-Combinator are even accepted, meaning that only one in every 200 companies that applies to Y.C. eventually succeeds."
Emphasis mine.
[+] http://koltai.co/notebook/the-risky-business-of-entrepreneur...
EDIT: Please disregard this post. Left for historical purposes. sama notes that the stats are inaccurate.
EDIT 2: The citations in that link I mention above are from nytimes and business insider. They might be accurate, if a bit old, but require verification. Treat them appropriately.