I am seeing about $200.00 US for the keypad, they sell a relay harness also if you need to use relays for your doors and rear hatch. If I can wire right into the door lock module on the hummer I do not think I would need any relays.
When I do the job I will post a little tutorial about it.
Some Suppliers:
Cruise N Comfort
A1 Electric - Torrance, CA
Mito Corp. Indiana