What is the fuel burn of the APU on the CRJ900?
It's surprisingly hard to come with that sort of data.  It's hard to come up with a fuel burn number because the fuel burn varies widely, because the loads vary widely, between electrical loads and bleed demand.  However, it's not too hard to come up with a theoretical maximum and estimate from that.
The best way to make an educated guess is to take the baseline HP rating of the Honeywell RE200 APU used in the CRJ7/900, around 325 hp, and work out the fuel burn at maximum power from the typical specific fuel consumption of turboshaft engines, which is somewhere around .5 to .6 lbs/hp/hr.  Say to be conservative, we'll use .6 lbs/hp/hr.
So wide open, the APU will be burning .6 x 325, or just under 200 lbs/hr.  It might get close to that if you are running all of the electrics off the APU gen and both packs.
Normally however, the demands are going to maybe 1/3 to 2/3rds of that.  If it's idle, it might be burning 30-50, and with bleed loads due to running the AC packs, maybe 100-150.
So I think you could use 100-125 lb/hr as a ballpark value when you want to allow for fuel used by the APU while on the ground and providing air conditioning and powering the electrical system. Or if you just want to assume the worst case and go by that, for an hour of sitting on the ground with the APU running everything, you can safely assume it's going to use not more than 200 lb of fuel.