The economics of the CRJ 200 are really bad, it wouldn't surprise me one bit that a 50 seat 700 is cheaper to operate. Obviously a 70 seat CRJ7 is better than a 50 seat CRJ7, but I would say with the premium seats, the CRJ7 at 50 easily beats a CRJ2. The fuel burn difference is only around 10%.
The 700 burns about 40% more in both climb and cruise. On taxi, the real world burn is only 10-20% higher since the 200 has to taxi with apu up. With its better climb, and slightly faster cruise speed, the trip fuel is 35-40% higher in a 700.
Lemme do some math seventy, divide by 50.... something like 1.4 yeah. I can’t speak to casm, but with respect to fasm (or gasm if you will), the 50 seater 200, and the 700 with 70 seats are nearly the same.
With 50 seats in it, the 700 should have approximately the fuel efficiency of a convair 880.