I am assuming that the CRX you are interested in has the factory B16A. This was not available here in the US. But TONS of people here have swapped this engine into US model CRXs. Mileage can vary widely. I have heard reports of getting anywhere from 26-34MPG on the freeway with this motor. Of course, it is hard to say what you might get. But keep in mind that virtually ALL the mileage reports on the higher end have been from swaps of 1992+ motors, with the car retrofitted to use the better 1992+ ECUs. So you might not see anything as high as 34MPG. On the other hand, few swappers here drive their cars very conservatively. So this could be a factor.
Since the gearing of the tranny is probably going to be a huge factor in mileage for that CRX, you may want to consider swapping in a b18a tranny. The b18a is a lower revving motor, unlike the be b16a, so it should keep your revs down much better while cruising on the highway.
To this same point, you may also want to look into swapping the "final gear" (think of it as the rear gear in a rear wheel drive car, the differential gear), since that would allow you to keep the b16a tranny, but you could still adjust the RPMs in all gears, to make you get better mileage.
The CRX is a great car to start with for MPG since it does very well in the aero aspect, and is a somewhat light car. Though it is nice, you may want to look for a CRX that doesn't have the DOHC motor and the glass roof. If you can find one with a SOHC and without the glass in the roof, it will be a lot lighter, and should make getting more MPG much easier.
On the never-ending quest for better gas mileage...