AlanB,
I have a 24x36 with the same construction. Around here that is the way that nearly all pole barns are constructed. They use "chicken house" trusses. A local chicken house builder will build a 40x60x12 with enclosed gables for $8000.00 Then you just enclose the sides yourself and pour a floor. Of course he will do a lock and key, but most people have him put up the roof, because he has a boom truck to set the trusses, then they finish the outside themselves.
I've got a buddy that has cows and he had the contractor I'm talking about build him a 40x60x14 with two 12x60 lean-to's. He basically has 64x60 underroof for $15,000.00. That doesn't include grading. My buddy is eventually going to close in the main 40x60 himself.
This type of construction is fairly simple. I set my trusses with my front end loader. Of course the trusses of a 40x60x12 on a 4/12 pitch requires a boom truck as it will be 19' from the ground to the ridge.
Chris