http://www.airlinemonitor.com/articles/8/part2/index.htm
I posted this before (right now the link is down, but it was up an hour ago so it should be back), but go to I believe pages 19-24 (on the left column) and compare CRJ900 (even 700) to the RJ85 (69 seats). Obviously the RJ85 is expensive as compared to them, however, look just at the hourly block costs of all of them versus the number of seats. I know, these are 2004 dollars, but it can be assumed that the cost increases due to fuel are shared across the board.

CRJ900 (total average) --

Hourly Block - $1768.70

Flight Crew Cost ASM - 0.64 cents

CASM - 6.7 cents

Cost per A/C mile - $5.20

Fuel cost per gallon - $1.21

AVERAGE SEATS PER A/C - 84.2

Total M/X hourly - 250.55

CRJ700 (total average) --

Hourly Block - $1425.69

Flight Crew Cost ASM - 1.3 cents

CASM - 6.26 cents

Cost per A/C mile - $4.34

Fuel cost per gallon - $1.28

AVERAGE SEATS PER A/C - 69.3

Total M/X hourly - $262.46

ARJ85 (total average) --

Hourly Block - $2001.73

Flight Crew Cost ASM - 1.35 cents

CASM - 10.53 cents

Fuel cost per gallon - $1.15

AVERAGE SEATS PER A/C - 69.0

Total M/X hourly cost - $460.83 (nearly twice as much for two more engines)

DC9 (all averaged) --

Hourly Block - $2897.26

CASM - 11.98 cents

Flight Crew Cost ASM - 3.46 cents

Fuel cost per gallon - $1.14

AVERAGE SEATS PER A/C - 104

Total M/X hourly cost - $479.84

NOW, using very simple math assumptions (I could be way off on this, but I'm trying atleast), 85 seats in the Avro is a 19 percent increase. So I decreased the CASM buy 19% and came up of with 8.53 cents. Yes, still higher than the CRJ900 and CRJ700, but that atleast buys a time stop gap until a suitable replacement and expansion of that seat capacity is possible. Plus, operating a single fleet with multiple hull sizes like Bombardier or Embraer offers increases an airlines strategic AND tactical flexibility for best matching route to capacity.