Developing an aircraft (or really any complex system) is not like someone sketches an aircraft and then some people build it, fly it and then more aircrafts are built.
You start with making concepts, then designs, which are prototyped or simulated. Then these designs are discussed over and over and revised.
And this is not only done for the whole aircraft, but for many components, e.g. fuselage, wings, landing gears, engines, avionics, mission systems, hardware and software.
If and only if all people involved are convinced that this is the most feasible solution in time, quality and cost, the actual manufacturing of the first aircraft takes place. And keep in mind that usually for an aircraft many parts are not just assembled, but also developed from scratch.
And all components are tested alone, then integrated and tested again until the whole aircraft is tested first on ground and then in flight.
So hundreds or thousands of people work for a couple of years for this aircraft while everyone earns 100k, so that's a real big chunk of the money.