depends on a lot of factors, but off the top of my head, i would offer 13k and see what they come back at.
Seems high to me. A dealer friend down the street has a nice looking 2014 Plug in Prius with 34,000 miles for $15,995. Lutz FL - 2014 Toyota Prius Plug-In Hybrid Navigation Camera Remote AC Heated Seats 34k Miles - Car for Sale - Tampa Hybrids Inc.
There will be variations due to condition, location, options, etc... But as per Kelley Blue Book, a private party seller (in my city) should list his car ranging from $13.7k (good condition) to $14.5k (excellent condition). Buying from a dealer will increase the book value of the vehicle. Again, as per KBB, a dealer should charge between $14.8k and $17k, depending on condition and options. A dealer certified Prius will command a premium. Kelley Blue Book With very limited knowledge about the car's condition, it seems to be priced high for a private party seller, and priced fairly for a dealer.
