It really sucks that ricers have put the Honda CRX on such a pedestal, as that would fit the bill. At your price range you could find one, but it would be a monstrous ripoff. It might still be worth a look considering your area. The CRX HF will easily get your target MPG, but be prepared to give up a lot of comforts you might have gotten used to in the Crown Vic', especially ride quality.
Another route, which may or may not be suitable, is to get a motorcycle. If you found the right one you could double your MPG target. Again, it would be a huge jump from the Crown Vic' and you would not be ale to drive it for almost half the year. Good luck whatever you decide.
BTW, I guess my '92 Jeep needs a tune-up because it is only getting about 18-24 mpg and even though they weigh about the same, my straight six should be doing better than that, I think.