Take the new car to CarMax and see what they'll offer for it. That would give you an absolute bottom line figure when selling it privately or trading it in. As suggested, selling privately should get you a better price. If you live in a state where trade-in value is deducted from the sales tax calculation for the new vehicle, take that into account too.
If getting rid of the car only nets $23k, and you spend $8k for a replacement, then you're saving $15k over the next few years. Maintenance costs will be less on something like a Fit (especially if you buy a stick shift). You didn't say what your current car is, but if it's German, then it will cost a lot as it ages.
When you start looking at replacement cars, make sure you get something that has been maintained. If it's due for brakes, tires, etc, make sure that work has been done as part of the deal. Watch out for dealers who put crappy 'new' tires on their cars.