Accodring to the Wiki, the Prius US sales were as follows for the first 4 years. The Volt's 2012 total could outsell prius' 2003, it's 4th year in US. And remember, the Prius came out in Japan in 1997, so it had a history to help sales in US when it was introduced here. A running start so to speak. Over 30k sold before it hit the shores of the US.
2000 5.6k
2001 15.6k
2002 20.1k
2003 24.6k
2000 5.6k
2001 15.6k
2002 20.1k
2003 24.6k