Comment by ak6te

The nature of science and exploration is that every mission is harder than the last, which is a reasonable first order proxy for expense. For NASA specifically, there has also been a large focus placed on making delivery to orbit and the moon a commercial service rather than something that NASA has to do in-house which has held us back.

And as others have said, there is really no such thing as a cheap rover. As a recent example, there was recently a push by NASA HQ to refurbish a Mars rover engineering unit (basically a one-to-one copy of the one sent to Mars, kept at JPL for use in ops planning and issue debugging) into an operational lunar rover. The estimated cost to take this existing piece of hardware and make it fully flight ready and suitable for the new mission was hundreds of millions of dollars.