Surprise, surprise...! Even a cheap plastic collector will not influence solar thermal system cost significantly...
While it's good to see serious studies of this topic, it should actually not surprise anyone in our industry: At least for small / individual systems, installation costs are dwarfing the costs of single components. Thus, any cost improvements in one component have little effect on the overall system costs.
Optimizing the whole system, and driving down installation costs are really key to low system cost.
The requirements and economics of large(r) systems are completely different, of course.