Having two rovers turned out to be insanely popular. Yes, it was expensive and NASA's budget is too small. But let's separate out the marginal cost here, because I'm pretty certain that if we had (for example) sent 4 rather than 2, it wouldn't necessarily have cost twice as much.
Many of us abide in disbelief of NASA's inability to leverage economies of scale. But like SpaceX and other private agencies have taught us, maybe this inability is a governmental-culture issue, not some sort of weird space-related issue.
