On a new construction install is there a huge difference in price between insulated box duct for both supply and return verses flex duct? Obviously there will be some due to more detailed planning and placement issues, but should the increase in cost be enough to discourage me from having that done?

Also is there a rule of thumb about max return lengths, etc? My current design has about a 75' run for return and probably close to 100' for the max supply. This is using one unit for 2400sqft, with the unit in a utility room in the garage vs an attic unit. I was planning on a 2 or 3 zone system.