I'm not sure of the weight of Jet fuel per gallon but gasoline is around 6lbs/gallon. So assuming:

6/lbs gallon

$3/gallon (roughly the price most US majors paid last quarter)

85%

LF on 50 seats.

The cost per revenue passenger for fuel would be

RJ: 2200/lbs / 6 x $3 / 43 (50 * 85%) = $25.58 per person

Q400 1650/lbs / 6 X $3 / 43 = $19.19 per person

(if we figure 85% of 76 seats in Q400 the cost drops to $12.69 per person.)

Either way the Q400 looks a lot cheaper on fuel burn basis, and if an airline could replace 2

RJ runs with one full Q400 then the savings really start to add up. Over a year if each aircraft made 6 roundtrips per day carrying 43 passengers with a delta of $6.39/passenger:

$6.39 x 43 x 12 (segments) x 330 days (allow for maintenance/down days) = $1,088,089

PER AIRCRAFT more expensive per year.

Before anyone jumps up and down, these are all theoretical numbers based on my assumptions, input your own assumptions, the overall answer will remain the same. A better comparison would be a CRJ700 vs Q400, that would give a better seat to seat comparison but I'm willing to bet there is a similar cost differential, maybe not quite as wide.