how much does it cost to get a new back deck on a 2005 nissan altima 2.5 sl?

Repairpal is not set up for body work estimates. It's impossible to give an estimate on body work without seeing the extent of the damage and being able to see if damage is superficial or more extensive. You will have to go to several body shops in person with the car so they can give you an estimate and advice. If you are not going through insurance tell them that straight off.