When you tell a team of scientist that budget only allows for a single launch in the next three decades, the cost of that telescope will inevitably be higher because they will want to optimize it and put as many instruments as possible. They will try to get as much science per kg as possible and verify a million times that nothing could ever fail. If deep space payload was a "cheap" commodity and the team designing the JWST were told they get to send 15 units instead of one, it would have been a much different situation.