There's no easy answer here - installation costs will depend on a number of factors including labor rates in your market and your specific requirements. An installation will require:
-mounting the unit (est. 1-2 hours)
-connecting to gas source (greatly varies by distance, est. 1-4 hours)
-connecting to electrical source (greatly varies by distance, est. 1-2 hours)
-venting the unit (est. 1-3 hours)
-connecting to controls (est. 1 hour)
Installation will usually require two workers, which can sometimes speed things up. Labor rates generally vary from $100-150/hr, depending on location. Some installations will require a lift, adding to the expense.