I think you can save much more purchasing direct from each vendor. Get you trusses from a local manufacturer, metal, lumber, etc. I would think the price difference would be about half - 2/3 the labor cost. Then just pay someone who does it day after day. Talk with you metal manufacturer about looking for an installer.
I have been looking at a pole barn (without sides). 24x36 kit of materials from the local package places is $2600. I can purchase seperate $1800. Just a thought.