my lfs charges $45 for 6" , i got a 12" arow healthy n he doesnt eat feeders or fish he live with 2" fish no problem n if u get 1 from an lfs they are feeed strictly feeders so i think a pellet trained one is worth something more, depends if the buyer wants it pellet trained or not
Average cost for 8-10" will be $50, you can consider buying a 5" for $15-25 if your other fish aren't too aggressive.
In asian aquarium stores, they usually sell arowana cheaper than in petco, petlands, etc... if not, you can also bargain.