With this job we replaced the elbows out of the side of the unit with a curb that allows us to attach the duct work from underneath the unit.  This allows for a cleaner application and less chance of a roof leak.  Include a curb, some ductwork is required and some code upgrades are common. On this 1,300 home this job cost $10,371. If we were to have used the existing elbows and avoided ductwork modifications the job would have been in the $5,000 range.