If you need extra prims to place demo vendors (it really is only one prim per products) then I think that should be covered in the price of the retail version and not by charging for a demo. There are few places that I will pay for something that will get thrown away after one use and the cost of those items is so high and the items such good quality that when I do go there I'm prepaired to fork out regardless of any cost. And I know there's a hair place that charges 5L$ for a demo.... I haven't been back there since I ran for the door.
For my own products I only put on a 1L$ price tag if I'm giving the proceeds for the item to charity. Otherwise I think its unnecessary and a deterent to potential customers. If people are doing it simply through force of habbit then they need to change with the times.