The best place to get duct loss coefficients is look in the SMACNA HVAC duct system design manual....it has all that info.
Costs?? call your local duct supply company...cost vary
I work in southern california...so the rule of thumbs might be different with certain requirments....I use 300 sqft/ton for exterior and 400 sqft/ton for interior zones....Air change rates you can find in the UMC or ASHRAE standards for that....and also for your ventilation requirements...In...
this system should be a VAV system....so there will be a diversity in your main duct sizing........As for the 3000 cfm box, if it serves an exterior zone....rule of thumb here in California is using a 65% diverstiy of the 3000 cold deck supply cfm to find the hot deck cfm....so if you have 3000...
