I'd think that if you're any kind of craftsman, professional or hobbist, and are willing to take the time to research grades of aluminum and aluminum stock, fasteners, hinges, tubbing and fastening devices, can find someone with the skills necessary to do what you can't do and get them to work for beer, maybe a little more, investigate types of materials and again, get a good awning company to work off line after hours, maybe, and take the time to design what you want, electrical connections, screens, heavy zippers and snaps, then you can probably come out with a really nice rig just for you for less than many of those rigs cost of the showroom floor.
You'r also paying for engineering (of some kind) design work, research and development, knowledge of materials and construction methods as well as the labor, overhead and materials. Most folks here probably could do such a project, but they may not have the time, energy or cash to pay for it as they go along, as such units can usually be financed as well.
As for me, I'd build one, but I'm kinda ahead of the game in many ways and it would still be a big PITA!