Any car can be expensive to fix, however, the Prius has a reputation for being problem-free. The high owner satisfaction rate backs this up. There is a lot of FUD that the detractors keep spewing, but Prius owners know that it's just FUD.
Of course, just like any used car you need to get it checked out for mechanical problems, prior accidents, and a clean title. There are services that can do this for you.
Join one of the Yahoo Prius groups and ask real Prius owners. You'll hear things like 300,000 trouble-free miles. My own 2001 and 2004 have been trouble-free and great cars to drive.
The "it will be expensive to fix" comes from people who don't own a Prius. In fact, the Prius is a very low maintenance vehicle. My 2004 has cost 12 cents per mile for dealer maintenance, tires, and fuel combined over the 120,000 miles I've driven it so far. The 2001 is similar but no logbook so I don't have the exact figures.
2004 Prius MPG from the logbook. (Complete years only):
2003-2004 -- 50.8 mpg 17,628 miles
2005 -- 52.6 mpg 14,688 miles
2006 -- 56.3 mpg 16174 miles
2007 -- 57.3 mpg 18384 miles
2008 -- 59.9 mpg 21755 miles
2009 -- 61.4 mpg 16177 miles
2010 -- 65.2 mpg 12134 miles
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