The costs don't scale quite as well as you'd expect. Actually building, testing, and transporting the thing is really expensive. If you build 2, it's still a lot of expensive bespoke parts. Maybe if you build dozens the economies of scale start to pay off

exactly my sentiment!

why not build two, given the fact that most of the budget is likely to be R&D costs.

Manufacture, certification and launch are still quite expensive. You'd easily run a couple extra billions building a fleet.

Having said that, I'd be delighted to have a fleet of them.