A major point missed by the other answers is that these rovers are exceptionally specialized for their mission, so building a second rover wouldn't be able to accomplish many additional scientific goals, without major modifications. The Curiosity rover and Perseverance share a lot of components, but pretty much the entire science suite is replaced or upgraded. As such, the expensive parts weren't shared, even though the huge value of the bus/bogie system and the skycrane were shared. Even the EDL systems have been changed a lot; the entry system on Perseverance does localization during some phases with a camera (a first for a spacecraft afaic, maybe the shuttle counts though since a human eyeball steered it to a runway). If you don't have additional science objectives that a second rover could accomplish, it will be very hard to justify that budget, even if it is only 20% more (for example).

This being said, clever mission design might take advantage of this cost structure where multiple units are cheaper to build, intending to build two cheaper spacecraft instead of one expensive one; eggs in two baskets. In this case, we've seen this happen with spirit and opportunity, which was a wildly successful program.