I am a project manager for a contractor in des moines, we build a ton of post frame buildings. i would recomend either the spray foam which should cost around $2.30 a sqft for a 8" thick wall. Another way to do it is to use an insulated panel for the living area, they are weather, air tight, and fast to install. you can also use traditional bat insulation inside the posts and install ridgid insulation between the exterior purlins, tape, caulk, and tyvek does a nice job sealing that up.. Spend the extra money on an energy heal truss, and seal up every possible place for air infiltration (thats the key)
If you need some more help or questions i can also assist you in finding suppliers and subcontractors just shoot me a pm.