This is a really great argument. While you can find used computers of all kind for $100, there is no guarantee what you will find in which condition. With the RP, you know you can buy a new machine for said price. This enables to plan availability as well as the curriculum, as the precise machine is known. This is the biggest value of the RP system anyway, the huge user base.

On the other side, it is perfectly reasonable to let the pupils find a screen on their own, most TVs or used computer screens will do.