Increasing production from 1 unit to 2 doesn't necessarily reduce costs. Due to demand and limited supply, for example, some prices increase. We know almost nothing about this thing. It's very speculative to say the cost would decrease. Also, how many space telescopes of this variety do we need?

All that said, it would be interesting to see NASA research on mass producing the more common components of its 'product line'. It does it for rockets, of course, but computers? Solar panels? Mars rover components? I'm sure it's been considered and I expect it's done in ways I'm not aware of.