I undrstand that OSB has gone out of sight in some places, but $1.40 per sqft installed? thats nearly $50 per sheet, so your whole labor cost in hiring a builder might be saved there alone! Around here 7/16 osb went thru the roof last year, to nearly $20 per sheet.... even considered selling a unit i had that i gave $8 per for, but i didnt..... now its back down a little.
$3.25 for metal roof per sqare foot? That too is a little high for a roof around here... even at a nickel per screw, the labor is too high in my estimation... maybe no one does metal roofs in your area....dont know. here I got colored metal, delta rib pattern for $1.10 per running foot in 24 inch coverage, and $1.60 for 36 inch coverage.... have not priced it since fuel went up this spring though... mightbe a quarter more. Screws go in 3 per row every 2 feet running, so a 10 foot run has 18 screws in pattern and a couple for the drip edge, or figure 25 cause you dropped a few and couldnt find them until you get a flat tire... $1.25 plus call it $12.50 for the metal, that covers 20 square feet, for $13.25.... your pre built wants 32.50 for that.... tkaes about 7 minutes to install that sheet of metal... or 9 per hour... $180. per hour of labor.... not bad for a part days work.... call a local builder....
And as far as the "showroom" model that you walked in... if I personally was gonna sell the public on this, it would be over built for that purpose, and any additions to the starter model would be on it so that the springyness of the floor was not there..... hucksters abound in all forms.... just because a person sees one thing it does not mean that the model he gets delivered will be built similar.... reputation or not.
Insulation, Look into bubble wrap under foil,
Tek Supply is a place to explore for a good deal or two once inawhile, and their tekfoil comes in a variety of sizes and r values. reflectex is another brand of the product.
It really is not difficult to build, And as bare sez, pour a slab, and when it is ready to build on you can getr donr in a weekend for the small size you are looking to build, and prolly for a whjole lot less than you think, even if you dont do anything but stand around and watch!
William