Honda Insight MPG Reports | Fuelly
Toyota Prius C MPG Reports | Fuelly
If you look at Fuelly for a larger sample size (and likely more typical numbers), you can see that the Insight can at least match the Prius C's average, but the Prius C can probably do better with any given driver.
However, if you get a 2010 Insight for $15000 and get 50MPG, vs a new Prius C for $20000 and get 60MPG, with gas at $4 a gallon, the Insight plus fuel would still put you ahead at 200,000 miles. The break even point would be about 400,000 miles, but probably less because of rising gas prices. So if you can get a cheap Insight, its a better deal as long as maintenance costs are similar. (even at $6 a gallon, the Insight is still cheaper at 200,000 miles)