Aware of the ground control system stuff, should be the same for both really. On checking the Avenger is stated to be 'fully compatible' with the Reaper's GCSs
Avenger price from here:
More drones, smaller Navy | UTSanDiego.com
Air Force buys Avenger drone from General Atomics | Deseret News
Reaper price from here:
Analysis of the Fiscal Year 2012 Pentagon Spending Request | COSTOFWAR.COM
However,
Factsheets : MQ-9 Reaper suggests $54M for 4 a/c including ground systems, albeit at 2006 prices.
Bottom line is there is no hard and fast info on accurate,
up to date figures; well, not that is necessarily independently verifiable.
Not trying to state any of the above as the absolutely irrefutable, happy to accept more accurate figures if available.