They did that with Spirit and Opportunity, which did cost a lot less. If money really were not an issue, I'm sure they'd like to send twin rovers again, and also pay for two rover teams, and for extra telecommunications capacity in the form of a dedicated orbiter[1] or two, and so on.
Good question. I imagine that communication infrastructure would have to be taken into consideration, but I don't know much of the details. The rover has the following capabilities for comms, but I don't know what the limits are in Mars orbit for relays:
There are allocations from the International Telecommunication Union for deep space communications in the S, X, K and Ka bands. This bandwidth (and parameters like transmit power of the relay spacecraft) effectively bounds how many bits per second you can get back from Mars. You can do better with optical communications and maybe some clever MIMO-like tricks.
If you had unlimited budget, a large stockpile of PU for the power supplies, and a dedicated DSN to not have to share time with other missions would it be possible to operate 10's or 100's of these rovers?
Not sure of the the weight but would it be possible to stack the rover payloads to send multiple rovers per launch?
Part of it is that they learn something from mission N and use that information to design the science instruments and choose the location for mission N+1. If N and N+1 launch simultaneously there's no chance for improvement in mission N+1.
[1] https://en.wikipedia.org/wiki/Mars_Telecommunications_Orbite...