Not gonna get into the discusion of which is better because thats not what the original questions asked and you'll never get a solid answer on that, they are different drugs and both have their pros and cons. Cost though according to our pharmacy that we replace our drugs at is as follow:
Fentanyl Inj 100mcg/2ml - $0.28
Mophine 2mg/ml carpuject - $0.89
Mophine 4mg/ml carpuject - $0.92
Mophine 10mg/ml carpuject - $0.91
Hope this helps. Let us know the outcome.