I honestly had no idea what a Challenger is worth so I looked on KBB and I'm seeing a "Very Good" condition 2010 Challenger SE with 50,000 miles has a trade-in value around $15,400 for the zip code I entered. Private party is listed as $17,200. I've owned a 3rd gen Prius (2010+) and the handling is much improved over the 2nd gen, plus it has a little more power thanks to the 1.8 liter engine versus the older 1.5 liter in the 2nd gen Prius. Since the OP is used to having a genuinely sporty car, the 3rd gen Prius would probably make the transition easier and feel less like he just moved about 7 rungs down the sportiness ladder of driving.
If the OP is looking to mainly trade straight across, I would say he could easily shop CPO Priuses in the $16-18k range and use his current trade-in as negotiation leverage. The dealer will (of course) say things like it's only worth $12k in trade-in, blah blah blah. If you run all the numbers and know you're offering a fair trade, then you simply stick to your guns and tell them that the only way to close the deal is to complete it as a trade in, or trade + X number of dollars. Then you are simultaneously working your own trade-in value as well as their set price on the Prius. Some dealers will also use No Negotiation pricing, so having the negotiation be about your trade-in gives you the leverage you need. I think this is a very valid tactic.
Another thing is that some places have unique registration or sales tax rules. For example in Arizona you must pay sales tax on car purchases. It's lower than the consumer goods rate of 9%, but still something like 6%. However if you have a trade in, that wipes out sales tax. So a $16,000 trade in wipes out $960 worth of sales tax. Suddenly your $16,000 car is really worth almost $17k just because of sales tax laws.
As for carmax, my sister has sold cars to them that split the difference between KBB trade-in and private party. I've been offered slightly below KBB trade-in on both cars I took to them, but ultimately took them up 1 time (I was buying a car from them, so I got that value of sales tax advantage) and sold the other private party. However the one I did private party they offered me $7000 and I got $9000 private party, negotiated down from $9500 list price. It's more work and more risk to sell cars private party, so that's a factor too. I'm not against doing it (obviously, as I have sold private party before) but I'm also a proponent of really weighing your options and not getting yourself mentally locked in to one route or another.
