Well 500 is a great price, but with a reciept, somethings wrong. He could have just wrote you an as is, no refunds reciept, or something like that. We have had folks try to load up buildings in the middle of the night, but without the proper equipment, its harder than it looks, especially with a sizeable building. However, if he used to do it, then he knows just how.
A 12x 15 of our make would run about 3500, non insulated. But its an all metal construction with a 2 year warrenty, mouse and waterproof. There are a lot of other companies around here that make them, but (without sounding too braggy)ours are the Cadillac of portable buildings. BTW, we also do doghouses, wellhouses, deer blinds, loafing sheds, custom stalls, bunkhouses, cabins, and custom metal trim fabrication and welding(another shameless plug lol).
BBH, if you ever wanted to build a shed, I'd be happy to offer any advice or pointers.