SpaceX is taking a very different approach from those of other launch companies, taking a far more rapid prototyping style approach to development, knowing that

SpaceX is taking a very different approach from those of other launch companies, taking a far more rapid prototyping style approach to development, knowing that the first few iterations are going to have problems that they can fix in the later versions. This sounds wasteful on paper, but in practice it is much much faster and amazingly enough cheaper. With traditional rockets you never make enough to enjoy economies of scale, but with SpaceX's approach they've dropped the cost of the engine by about an order of magnitude over just a few years, and it was already a very inexpensive engine. For the cost of a single RS-25 you can buy 400 Raptors.